Add YOPT_S_DEFER_EVENT_CB for user custom event dispatch

Benefit: User can do custom packet resolve at network thread, such as decompress  and crc check.

### callback prototype: 
```cpp
typedef std::function<bool(event_ptr& event)> defer_event_cb;
```

### callback return value
true: io_service will continue enque to event queue
false: io_service will drop the event