Ping/Pong Extensions

[ScottyPoi]

Implements changes in ethereum/portal-network-specs#348

Redefines the CustomPayload of PING and PONG messages to include a type parameter.

This allows different types of PING messages to provide / request different kinds of data from a peer.

Currently this includes:

• type 0: Client Info and Capabilities
• type 1: Basic Radius
• type 2: History Radius

Clients will respond with a PONG of the same type, or:

• type 65535: Error Response

[acolytec3]

Looks good so far! Couple of relatively small things in comments.

Other feedback already noted on discord.

[acolytec3]

Looks good to me overall though I question whether we need a new dependency to grab the commit hash. I think we can safely assume that anybody running this client locally will either be running it from a cloned git repo or in docker and I'm pretty sure git is installed in our docker image so we should able to just do an exec('git rev-parse HEAD') or something to that effect and read the output instead of requiring a new dependency for this. I know I'm being persnickety but just my 2 gwei. Happy to be persuaded otherwise.