BaseMsgReadWriter
- Change `BaseMsgReadWriter` to encode/decode messages with abstract method, which can be implemented by the subclasses. This allows us to create subclasses `FixedSizeLenMsgReadWriter` and `VarIntLenMsgReadWriter`.
SecureSession
Make security sessions(secio, noise) share the same implementation `BaseSession` to avoid duplicate implementation of buffered read.
NoiseConnection
TODO: Add a buffer to read only `n` bytes in `read(n)`
