Add a version string to all outgoing messages

[ssiloti]

BEP 5 has been updated to document the long standing de-facto standard of including a version string in RPC messages. This is an important feature for identifying implementations which may be lacking features or misbehaving. The relevant section of BEP5:

A key v should be included in every message with a client version string. The string should be a two character client identifier registered in BEP 20 followed by a two character version identifier.

The convention is for the client identifier to identify the DHT implementation rather than the client application. Thus the same identifier should be used regardless of which client is using this module.

[nictuku]

I'd gladly accept a PR that implements this. Thanks