Fixes abour replication lag

- Do not terminate master replication session until it receives all slave's ACK
- Handle slave's ACK received out of order.
Only raise updated replication lag event when the lag is modified
Fix IT test flakiness (on my machine!)
Make Jerome happy with a new whitespace
Merge pull request #292 from wajam/manu-replication-lag-fix

Fixes about replication lag