LPProtocol should have `method handler`

[gmelodie]

Original comment:

code all over the place does this assigns like this. would be better if there was handle function so that code is
kad.handler = kad.handlerImp, this makes constructor new much more concise and shorter.
then if code goes down this route, why handlers are not just some proc/method on type?

In other words, we should probably change LPProtocol to include a handler method that should be implemented by protocols

method handler*(conn: Connection, proto: string) {.base, async: (raises: [CancelledError]).} =