Renamed 'bytes' and 'msg' to 'payload' in all Message classes

Renamed 'bytes' and 'msg' to 'payload' in all Message classes. Also, some minor comment fixes.

* This is intended to clarify distinction between full message and its payload to match terminology advertised on https://en.bitcoin.it/wiki/Protocol_specification
* Does not change public API.
* Refactoring was mostly automated and should not introduce bugs. All tests pass.