Fixes to make the galera.MDEV-20793 test deterministic.
Specifically, after --send COMMIT, there is now a sync point to catch a known state of the COMMIT execution
Assertion failed in wsrep-lib after transaction replay which
failed due to conflict in certification.
- Implemented reproducible test case MDEV-20793 to reproduce the crash.
- Fixed wsrep-lib to deal with certification error during replay.