MDEV-6247: Merge 10.0-galera to 10.1.

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.
Merge branch 'bb-10.1-galera' of github.com:MariaDB/server into bb-10.1-galera