Clear event watchers before invoking connection callback

This lets a socket remain "Idle" without receiving readability events.
Useful for an immediately-released connmgr socket

Change-Id: If57d6ac4adc3da48a0ac2b35f9cfe1c15eb07087
Reviewed-on: http://review.couchbase.org/33007
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>