MB-24426: Use virtual destructors for some of the DCP classes

MB-24426: Use virtual destructors for some of the DCP classes

DCP classes like 'DCPProducer', 'DCPConsumer', 'Consumer',
'ActiveStream' and 'PassiveStream' have subclasses derived from them.
It is appropriate to use virtual destructors to avoid any memory
leaks during object deletion.

Change-Id: I8b8457095b7c9914ca2c3d4b18a2251833e6c770
Reviewed-on: http://review.couchbase.org/78334
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>