refactor peers.py & packeter.py

[heikoheiko]

• currently serialization is done in packeter.py; interpretation and casting of deserialized data is (partially) done in peers.py; networking code is spread over two files which makes development complicated and error prone
  • serialization code should exist next to the deserialization code in peer.py
• wire protocol documentation in docstrings went missing after refactoring
  • re-add wire protocol documentation
• blocks and transactions are exchanged as rlp.decode(obj.serialized())
  • de/serialization should accept and return Transaction and Block instances

[heikoheiko]

introduced blocks.TransientBlocks as a start