Potential memory leak due to cyclic reference between DcpProducer and ActiveStream objects

DcpProducer obj holds a reference to an ActiveStream obj and vice versa.
We will have a memory leak if the reference to the ActiveStream is not released explicitly (by DcpProducer obj) when the higher level DcpConnMap obj decides that it no longer needs DcpProducer obj.

Currently if DcpConnMap obj gets deleted without calling shutDownAllConnections() we can have memory leaks. This is demonstrated by the address sanitizer failure in http://review.couchbase.org/#/c/84652/1