[BP] MB-16915: Remove cyclic reference between DcpConsumer and PassiveStream.

[BP] MB-16915: Remove cyclic reference between DcpConsumer and PassiveStream.

DcpConsumer holds a reference to PassiveStream and vice versa. We must
make sure that one of them (DcpConsumer here) releases the reference
to another in a function other than the object destructor.

Change-Id: I8e5c262bc5ac50342f85ba80d481987a26a7a21d
Reviewed-on: http://review.couchbase.org/57429
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-on: http://review.couchbase.org/57449
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>