CCBC-155 Fix multi-observe during server failures

Change-Id: I722011986051fc1ab0921ffd3708f5967bedf06d
Reviewed-on: http://review.couchbase.org/23527
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>