ctdb-mutex: Change parent checking to use an asynchronous computation

ctdb-mutex: Change parent checking to use an asynchronous computation

Put the checking for the process being immediately re-parented into
the computation too.  This will be very rare and doing it
consistently makes testing saner.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>