Add discovery v5 support

[gsalgado]

Current implementation doesn't capture discv5 frames. ISTM dissect_ethereum_heur() returns false for them, but we'd also need to add support for the v5 commands

[gsalgado]

I'll have a go at this myself if nobody has started on it yet

[raulk]

@gsalgado – great! If you need help, just post here :-)

[gsalgado]

@raulk #18 has a work in progress for that, which can dissect most v5 commands, feedback welcome!

[raulk]

@gsalgado took a first look and posted a few comments. Thanks!