MB-8378 Clean up all the TAP connections within a destroy thread

As a separate dedicated thread performs destroying an engine
instance, we can simply clean up all the TAP connections within
that destroy thread. In this way, it doesn't need to wait for
NON-IO dispather to complete cleaning up TAP connections.

Change-Id: I39f3373b2867a2357a4f802bdc4e6ba77f4f5c82
Reviewed-on: http://review.couchbase.org/26625
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>