MB-8175: Fix tap implicit acks stream unit test failure

We should only decrement the transmitted stat if we actually have
the corresponding tap message in the tap log.

Change-Id: I60fd59419043b7e2af87694dbac5980a5f0ecc72
Reviewed-on: http://review.couchbase.org/25977
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>