Usage:
1. Register handlers in incoming port with nxt_port_rpc_register_handler().
2. Use return value as a stream identifier for next nxt_port_socket_write().
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.
New port message type introduced NXT_PORT_MSG_REMOVE_PID. Default handler
removes process description from nxt_runtime_t with all ports, incoming and
outgoing mmaps etc.
There is a case in router where we use port in router connection thread.
Buffers are allocated within connection memory pool which can be used only in
this router thread. sendmsg() can be postponed into main router thread and
completion handler will compare current engine and post itself to correct
engine.
- 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);
