Drainer: Support TiDB to Oracle Replication

[cartersz]

When users migrate their Oracle databases to TiDB, they need a way back, a survival plan that escapes from the failed TiDB, thus we need a solid solution of TiDB to Oracle data replication.
Currently there's no solution of TiDB to Oracle replication (T2O). Some of the customers deployed an unstable setup - T2M2O, which uses MySQL as a relay, the problem is TiDB and MySQL are not compatible, plus this replication chain is too long.

This enhancement of drainer could directly replicate from TiDB to Oracle, the replication chain is short, which means it's more stable than T2M2O solution. And there's no other MySQL involved which is a bottleneck of the performence.

1. dml data migrate to oracle. see: replicate dml statements in binlog to oracle #1094
2. 'truncate table' ddl statement to oracle. see: replicate ddl data in binlog to oracle #1095
3. saving check point to oracle. see adaptation to oracle checkpoint #1097
4. relay log for oracle. see relay log for oracle #1104