s3:g_lock: try to keep the watch instance during g_lock_watch_data()

s3:g_lock: try to keep the watch instance during g_lock_watch_data()

Unless the unique_lock_epoch changes via g_lock_lock()/g_lock_unlock()
we try to keep our existing watch instance alive while waiting
for unique_data_epoch to change.

This will become important in the following commits when the
dbwrap_watch layer will only wake up one watcher at a time
and each woken watcher will wakeup the next one. Without this
commit we would trigger an endless loop as none of the watchers
will ever change unique_data_epoch.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15125

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>