get_blockchain_synopsis throws when reference_point is in the future

[theoreticalbts]

This ticket is for a bug in get_blockchain_synopsis() which causes it to throw when reference_point is in the future (basically it calls is_included_block() which calls get_block_id_for_num() which throws if the blocknum isn't in the local block DB). The p2p code isn't expecting a throw there and somehow it ends up causing the peer to disconnect.

[theoreticalbts]

The fix here is to add a find_block_id_for_num() function which indicates failure by returning block_id_type() instead of throwing, then use that at the problematic call site in is_included_block().

To cut down on duplicate code, I'm going to also re-implements get_block_id_for_num() by calling find_block_id_for_num() and throwing if the result is block_id_type().

[theoreticalbts]

Relevant log messages from production:

Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]: 1923092ms p2p        message_oriented_connection.cpp:188 read_loop            ] message transmission failed 10 assert_exception: Assert Exception
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]: results.size() == 1: 
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]:     {}
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]:     th_a  database.cpp:257 get_block_id_for_num
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]: 
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]:     {"block_num":12644205}
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]:     th_a  database.cpp:261 get_block_id_for_num
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]: 
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]:     {}
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]:     th_a  application.cpp:820 get_blockchain_synopsis
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]: 1923092ms p2p        message_oriented_connection.cpp:205 read_loop            ] disconnected 10 assert_exception: Assert Exception
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]: results.size() == 1: 
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]:     {}
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]:     th_a  database.cpp:257 get_block_id_for_num
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]: 
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]:     {"block_num":12644205}
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]:     th_a  database.cpp:261 get_block_id_for_num
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]: 
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]:     {}
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]:     th_a  application.cpp:820 get_blockchain_synopsis
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]: 1923092ms p2p        node.cpp:2981                 on_connection_closed ] Remote peer 212.47.249.84:40696 closed their connection to us
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]: 1923092ms p2p        message_oriented_connection.cpp:286 destroy_connection   ] in destroy_connection() for 
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]: 1923092ms p2p        message_oriented_connection.cpp:299 destroy_connection   ] Exception thrown while canceling message_oriented_connection's read_loop, ignoring: {"code":0,"name":"exception","message":"unspecified","stack":[{"context":{"level":"warn","file":"message_oriented_connection.cpp","line":207,"method":"read_loop","hostname":"","thread_name":"p2p","timestamp":"2017-06-08T15:32:03"},"format":"disconnected: ${e}","data":{"e":"10 assert_exception: Assert Exception\nresults.size() == 1: \n    {}\n    th_a  database.cpp:257 get_block_id_for_num\n\n    {\"block_num\":12644205}\n    th_a  database.cpp:261 get_block_id_for_num\n\n    {}\n    th_a  application.cpp:820 get_blockchain_synopsis"}}]}
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]: 1923092ms p2p        message_oriented_connection.cpp:286 destroy_connection   ] in destroy_connection() for 
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]: 1923092ms th_a       websocket_api.cpp:88          on_message           ] message: {"id":50035,"method":"get_block","params":[12643559]} 
Jun 08 11:32:03 steemit-production-steemd-001_i-0141d0a9b911f945d messages: Jun  8 15:32:03 ip-172-31-12-104 docker/be993797e8f2[4378]: 1923092ms p2p        message_oriented_connection.cpp:299 destroy_connection   ] Exception thrown while canceling message_oriented_connection's read_loop, ignoring: {"code":0,"name":"exception","message":"unspecified","stack":[{"context":{"level":"warn","file":"message_oriented_connection.cpp","line":207,"method":"read_loop","hostname":"","thread_name":"p2p","timestamp":"2017-06-08T15:32:03"},"format":"disconnected: ${e}","data":{"e":"10 assert_exception: Assert Exception\nresults.size() == 1: \n    {}\n    th_a  database.cpp:257 get_block_id_for_num\n\n    {\"block_num\":12644205}\n    th_a  database.cpp:261 get_block_id_for_num\n\n    {}\n    th_a  application.cpp:820 get_blockchain_synopsis"}}]}

[theoreticalbts]

Update: This patch seems to have changed the semantics. Let's bring it back to the old semantics that simply returns the ID on the main chain that ignores forks.

As @mvandeberg pointed out, we need at least two queries here. One needs to be an in-memory query for reversible blocks, the other needs to be a block DB query for irreversible blocks.

I'm actually going to implement three queries, one to the TAPOS buffer which is constant-time and usually succeeds for reversible blocks, one to the block DB for irreversible blocks, and a final check to walk backwards in the fork DB.

The logic is the same as in update_last_irreversible_block() so I'll factor it out.

Related: #844.