Imp: support batch write

[wongoo]

Package is write one by one currently, it can be changed to batch write to improve the performance.

The key points of design:

1. add a []byte response for Writer.Write, which can be aggregated to write in batch：

type Writer interface {
  // if @Session is udpGettySession, the second parameter is UDPContext.
  Write(Session, interface{}) ([]byte, error)
}

2. enable user to config to use batch or single write mode.
3. aggregate bytes of packages to write in batch, set default batch size to 10.

[wongoo]

ref: #26