ctdb-readonly: Avoid a tight loop waiting for revoke to complete

ctdb-readonly: Avoid a tight loop waiting for revoke to complete

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12697

During revoking readonly delegations, if one of the nodes disappears,
then there is no point re-trying revoking readonly delegation immedately.
The database needs to be recovered before the revoke operation can
succeed.

However, if the revoke is successful, then all the write requests need
to be processed immediately before the read-only requests.  This avoids
starving write requests, in case there are read-only requests coming
from other nodes.

In deferred_call_destructor, the result of revoke is not available and
deferred calls cannot be correctly ordered.  To correctly order the
deferred calls, process them in revokechild_destructor where the result
of revoke is known.

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