CCBC-150: Fix callback handling for multiple observe requests

Change-Id: I32b63ed99a3bfc238d72d5c60d048ca88090dca4
Reviewed-on: http://review.couchbase.org/23526
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>