SPY-155: Synchronize when future listeners are notified.

This changeset prohibits a possible race condition when listeners
are called from the callback. Proper sync has been in place for
both adding and removing, but not for notifying.

Change-Id: I4cfa0036c2b7b89985b0026c5da5dca26cc9fe52
Reviewed-on: http://review.couchbase.org/33800
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>