Optimized send message allocations.

For empty write queue cases, it is possible to avoid allocation and enqueue
send message structures.  Send message initialized on stack and passed to
write handler.  If immediate write fails, send message allocated from engine
pool and enqueued.

src/nxt_port.h

@@ -118,8 +118,6 @@ typedef struct {
     nxt_port_msg_t      port_msg;
 
     nxt_work_t          work;
-    nxt_event_engine_t  *engine;
-    nxt_mp_t            *mem_pool;
 } nxt_port_send_msg_t;