The messages.py module should contain code and helpers for
[de]serializing p2p messages. Specific usage of those messages should
be in p2p.py. Therefore move MY_VERSION to p2p.py.
Also rename to P2P_VERSION to distinguish it from
other versioning used in Bitcoin/Bitcoin Core.
Also always set the nVersion field in CBlockLocator to 0 and ignore the
field in deserialized messages. The field is not currently used for
anything in Bitcoin Core.