MB-4460: Don't aggregate a disconnected tap stream

When a tap stream is disconnected and then reconnected we create
a dummy tap stream with an anonymous name which is later cleaned
up. These tap connections should not be included in the tap agg
stats.

Change-Id: I044298a55555b803bad67fcf7e033ff6adc57be2
Reviewed-on: http://review.couchbase.org/25409
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>