This commit changes the discovery protocol to use the new "v4" endpoint
format, which allows for separate UDP and TCP ports and makes it
possible to discover the UDP address after NAT.
As of this commit, p2p will disconnect nodes directly after the
encryption handshake if too many peer connections are active.
Errors in the protocol handshake packet are now handled more politely
by sending a disconnect packet before closing the connection.
This mostly changes how information is passed around.
Instead of using many function parameters and return values,
put the entire state in a struct and pass that.
This also adds back derivation of ecdhe-shared-secret. I deleted
it by accident in a previous refactoring.
The diff is a bit bigger than expected because the protocol handshake
logic has moved out of Peer. This is necessary because the protocol
handshake will have custom framing in the final protocol.
