* p2p/discover: add more packet information in logs

This adds more fields to discv5 packet logs. These can be useful when
debugging multi-packet interactions.

The FINDNODE message also gets an additional field, OpID for debugging
purposes. This field is not encoded onto the wire.

I'm also removing topic system related message types in this change.
These will come back in the future, where support for them will be
guarded by a config flag.

* p2p/discover/v5wire: rename 'Total' to 'RespCount'

The new name captures the meaning of this field better.