Moving nxt_stream_ident to shared memory.

This aims to avoid stream id clashes after router restart.

src/nxt_port_rpc.h

@@ -11,6 +11,8 @@
 typedef void (*nxt_port_rpc_handler_t)(nxt_task_t *task,
     nxt_port_recv_msg_t *msg, void *data);
 
+nxt_int_t nxt_port_rpc_init(void);
+
 uint32_t nxt_port_rpc_register_handler(nxt_task_t *task, nxt_port_t *port,
     nxt_port_rpc_handler_t ready_handler, nxt_port_rpc_handler_t error_handler,
     nxt_pid_t peer, void *data);