Suppress unchecked warnings on completion listeners.

Change-Id: Icf7af9df6b7a1fa3df6892ec0a3b3e4175f86d33
Reviewed-on: http://review.couchbase.org/30375
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>