Throw decoding exception if there's an invalid node distance

[jtraglia]

PR Description

According to the spec, there are 256 buckets and 0 is reserved for our node, so a distance value can be in [0, 256].

For each 0 ≤ i < 256, every node keeps a k-bucket for nodes of logdistance(self, n) == I. The Node Discovery Protocol uses k = 16, i.e. every k-bucket contains up to 16 node entries.

I see that this is enforced in the KBuckets class, but I think we should throw a decoding error if we receive a FindNodeMessage with an invalid distance. We could just ignore that distance, but I think we should ignore the message:

https://github.com/ConsenSys/discovery/blob/d780ba4642db8acd1a6a8253d9e86018eafb8b89/src/main/java/org/ethereum/beacon/discovery/storage/KBuckets.java#L22-L29

Also, in my opinion, the spec is pretty vague about this detail. My first impression told me that a distance of 256 is invalid, but after I thought more about it, it seems valid. I'm still kind of confused though 🤷

[ajsutton]

LGTM. I think you're right that 256 is a valid distance (and also agree that the spec is unclear...)