inconsistent semisync results

Inconsistent results were discovered when testing the performance of semi-synchronous replication - more precisely: how fast the master can process certain write transactions.

The benchmark should show which variant is faster:

• rpl_semi_sync_master_wait_point = AFTER_SYNC or
• rpl_semi_sync_master_wait_point = AFTER_COMMIT

It was expected that after commit would be slower because in this mode the slave has much more to do before it signals ACK to the master which in turn sends ACK to the client. So the client has to wait longer for the COMMIT to finish and the throughput would be lower.

Instead the observed behavior was the opposite: whenever there was a difference between after sync and after commit, after commit was faster. Further more: severe slave lag was detected even for wait point = after commit. The slave lag was monitored by sampling @@gtid_current_pos of master and slave and comparing the sequence number of it.