Abstract out NetworkStream

This introduces a new Trait, NetworkStream, which abstracts over
the functionality provided by TcpStream so that it can be easily
mocked and extended in testing and hyper can be used for
other connection sources.
Use trait objects and dynamic dispatch to abstract over NetworkStream

Server and client benchmarks show that this makes very little difference
in performance and using dynamic dispatch here is significantly more ergonomic.

This also bounds NetworkStream with Send to prevent incorrect implementations.

Allows the implementation of mock streams for testing and flexibility.

Fixes #5