JCBC-350: One more spot where to fix the race condition.

This changeset also fixes the possible inconsistency in the observePoll
method, not only in observe itself.

Change-Id: Ifbec6e88e2df534724e48a173413ec41d6f35175
Reviewed-on: http://review.couchbase.org/28933
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>