Now all galera_sr tests succeed when SR speedup is disabled
and most galera_sr MTR tests succeed when SR speedup up is enabled.
The failing tests are listed in "skipped_tests" file.
The following MTR test suite run succeeds:
(cd mysql-test; ./mysql-test-run.pl --mysqld="--wsrep_SR_store=undolog"
--suite=galera_sr --skip-test-list=../skipped_tests --force)
* Add method `restore_prepared_transaction` to `client_state` class
which restores a transaction state from storage given its xid.
* Add method `commit_or_rollback_by_xid` to terminate prepared XA
transactions by xid.
* Make sure that transactions in prepared state are not rolled back
when their master fails/partitions away.