Issue: XA PREPARE, XA ROLLBACK and XA COMMIT for transaction
from a disconnected session does not take global commit lock
and modifies binlog and innodb redo log even when server is
in FTWRL. This could lead to inconsistent backups when
backup tools assume the server is in read-only state during
FTWRL mode.

Fix: Take global commit lock during these operations. Failure
to acquire lock during XA PREPARE will result in transaction
rollback. Failure to acquire lock during XA COMMIT and XA
ROLLBACK will return an error without changing transaction state.