lib: Add g_lock_watch_data_send/recv()

lib: Add g_lock_watch_data_send/recv()

Same concept as dbwrap_watched_watch_send/recv: Get informed if the
underlying data of a record changes. This utilizes the watched
database that g_lock is based upon anyway. To avoid spurious wakeups
by pure g_lock operations this patch adds a sequence number for the
data that is stored in the g_lock data field.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>