[2sp] elections: new `box.info` field and binary protocol changes

[TarantoolBot]

Product: Tarantool
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_info/election/
https://www.tarantool.io/en/doc/latest/dev_guide/internals/box_protocol/
SME: @ sergepetrenko

Details

• box.info.election got a new field: leader_idle.
  When elections are enabled, it shows time in seconds since the last
  interaction with the known leader.

• IPROTO_RAFT request got 2 new fields:
  IPROTO_RAFT_LEADER_ID = 0x04 - uint - the id of the current leader
  (as seen by the node which issued the request).
  IPROTO_RAFT_IS_LEADER_SEEN = 0x05 - bool - whether the node has a
  direct connection to the leader.

Requested by @sergepetrenko in tarantool/tarantool@421a096.