With wsrep_gtid_mode=ON, the appropriate commit hooks were not
called in all cases for applied streaming transactions.

As a fix, removed all special handling of commit order critical
section from Wsrep_high_priority_service and Wsrep_storage_service.
Now commit order critical section is always entered in ha_commit_trans().

Check for wsrep_run_commit_hook is now done in handler.cc, log.cc.
This makes it explicit that the transaction is an active wsrep
transaction which must go through commit hooks.