NCBC-693: ODE in ConnectionPool.Dispose when run against cluster_run

Change-Id: I078b7038b147b352379d0f708e1cb213b63c7858
Reviewed-on: http://review.couchbase.org/42863
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>