MB-7435: Fix bug related to tap cursor deregistration during backfill

When we schedule backfill we deregister the tap cursor for a specific
tap stream and as a result any backfilled tap stream would have an
incorrect estimate since we search for tap streams based on tap
cursors.

Change-Id: I16a4f8b8e9bc8eb05de0f3fe76c7babd8e0810df
Reviewed-on: http://review.couchbase.org/25885
Reviewed-by: Jin Lim <jin@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>