ctdb: add/implement CTDB_CONTROL_TCP_CLIENT_DISCONNECTED

ctdb: add/implement CTDB_CONTROL_TCP_CLIENT_DISCONNECTED

With multichannel a ctdb connection from smbd may hold multiple
tcp connections, which can be disconnected before the smbd
process terminates the whole ctdb connection, so we a
way to remove undo 'CTDB_CONTROL_TCP_CLIENT' again.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15523

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Martin Schwenke <martin@meltin.net>
(cherry picked from commit c6602b686b4e50d93272667ef86d3904181fb1ab)