MDEV-6656: Test wsrep.variables hangs
Analysis: wsrep_applier_thread shutdown signaling does not always work
correctly causing a timing problem where main thread is waiting in a
condition variable a signal that all worker threads to end.
Merged lp:maria/maria-10.0-galera up to revision 3879.
Added a new functions to handler API to forcefully abort_transaction,
producing fake_trx_id, get_checkpoint and set_checkpoint for XA. These
were added for future possiblity to add more storage engines that
could use galera replication.
