Go: fixing racing condition on app start.

Request procesing loop should be started in ready handler to avoid race
between go-routine start and context ready flag assignment.

The issue introduced in 43553aa72111.

go/nxt_cgo_lib.c

@@ -30,6 +30,7 @@ nxt_cgo_run(uintptr_t handler)
     init.callbacks.port_send       = nxt_cgo_port_send;
     init.callbacks.port_recv       = nxt_cgo_port_recv;
     init.callbacks.shm_ack_handler = nxt_go_shm_ack_handler;
+    init.callbacks.ready_handler   = nxt_go_ready;
 
     init.data = (void *) handler;