The user XA commit execution branch was caught not have been covered
with MDEV-21953 fixes.

The XA involved deadlock is resolved now to apply the former fixes
pattern.
Along the fixes the following changes have been implemented.
- MDL lock attribute correction
- dissociation of the externally completed XA from the current
  thread's xid_state in the error branches
- cleanup_context() preseves the prepared XA
- wait_for_prior_commit() is relocated to satisfy both
  the binlog ON (log-slave-updates and skip-log-bin)
  and OFF slave execution branches.