nxt_req_conn_link_t still used for lookup connection by request id.
New nxt_req_app_link_t (ra) allocated from conn->mem_pool using mp_retain().
ra stored in app->requests if there is no free worker to process request.
- request to connection mapping in engine;
- requests queue in connection;
- engine port creation;
- connected ports hash for each process;
- engine port data messages processing (app responses);
