About this change #
In this change, gnet integrates multiple codecs to encode/decode network frames into/from TCP stream: LineBasedFrameCodec, DelimiterBasedFrameCodec, FixedLengthFrameCodec and LengthFieldBasedFrameCodec, referencing netty codec,
in addition to these built-in codecs, gnet also supports customized codecs by functional option Codec.
