increase code coverage of test cases
add SimpleProtoCodec for TcpConnectionSimple

fix a data race warning when use go test -race

gix a waitgroup problem when manual call TcpListener.Close

fix a leak problem

简化proto消息注册接口

PacketHeader改为interface,以便于扩展自定义消息结构
新增不使用RingBuffer的TcpConnectionSimple
新增自定义消息结构的示例