The fix of MDEV-23328 introduced a background thread for
killing conflicting transactions.
Thanks to the refactoring that was conducted in MDEV-24671,
the high-priority ("brute-force") applier thread can kill the
conflicting transactions itself, before waiting for the
locks to be finally released (after the conflicting transactions
have been rolled back).

This also allows us to remove the hack LockGGuard that had to
be added in MDEV-20612, and remove Galera-related function
parameters from lock creation.