MB-22738: Move stripping of value from DCP stream back into ep-engine

MB-22738: Move stripping of value from DCP stream back into ep-engine

A revert of the http://review.couchbase.org/#/c/72398/
with the addition that determining whether whether to retrieve only
is made on the connection level, (as opposed to the stream level).

Change-Id: I641978c2be6c67e6a9d96c0a229ff21688c74055
Reviewed-on: http://review.couchbase.org/76418
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Jim Walker <jim@couchbase.com>