The hang can happen between a lock connection issuing KILL CONNECTION for a victim,
which is in committing phase.
There happens two resource deadlockwhere  killer is holding victim's
LOCK_thd_data and requires trx mutex for the victim.
The victim, otoh, holds his own trx mutex, but requires LOCK_thd_data
in wsrep_commit_ordered(). Hence a classic two thread deadlock happens.

The fix in this commit changes innodb commit so that wsrep_commit_ordered()
is not called while holding trx mutex. With this, wsrep patch commit time mutex
locking does not violate the locking protocol of KILL command
(i.e. LOCK_thd_data -> trx mutex)

Also, a new test case has been added in galera.galera_bf_kill.test for scenario
where a client connection is killed in committting phase.