PG,ReplicatedPG: handle_watch_timeout must not write during scrub/degraded

PG,ReplicatedPG: handle_watch_timeout must not write during scrub/degraded

Currently, handle_watch_timeout will gladly write to an object while
that object is degraded or is being scrubbed.  Now, we queue a
callback to be called on scrub completion or finish_degraded_object
to recall handle_watch_timeout.  The callback mechanism assumes that
the registered callbacks assume they will be called with the pg
lock -- and no other locks -- already held.

The callback will release the obc and pg refs unconditionally.  Thus,
we need to replace the unconnected_watchers pointer with NULL to
ensure that unregister_unconnected_watcher fails to cancel the
event and does not release the resources a second time.

Signed-off-by: Samuel Just <sam.just@inktank.com>