MB-21303 Handle ack from dcp producer for control_request correctly

control_request for flow control buffer size is added into the
pending request dictionary with nil as SendTo parameter as
gen_server need not reply when it gets response from producer.
This was not handled correctly when stream_response is handled
in the client which is fixed in this patch.

Change-Id: I7afb11adbde79fe2ee0895feac8fec91d563a062
Reviewed-on: http://review.couchbase.org/68525
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Abhishek Singh <abhishek@couchbase.com>
Tested-by: Abhishek Singh <abhishek@couchbase.com>
Tested-by: buildbot <build@couchbase.com>

src/couch_dcp/src/couch_dcp_client.erl

@@ -488,8 +488,6 @@ handle_info({get_all_seqs, From}, State) ->
 % Reply back to waiting callers
 handle_info({stream_response, RequestId, Msg}, State) ->
     State3 = case find_pending_request(State, RequestId) of
-    {_, nil} ->
-        State;
     {ReqInfo, SendTo} ->
         State2 = case ReqInfo of
         {add_stream, PartId} ->