fix: re-enable ensuring queries run along disjoint paths

[zeroxbt]

I noticed that during the getClosestPeers operation, the query paths might overlap and the same nodes will be queried multiple times.
There is no need to query a node if it has been queried during the execution of another query path, so the peersSeen set should be shared among different query paths belonging to the same query.

[wemeetagain]

LGTM

@achingbrain should also have a look

[achingbrain]

Why has this been removed? Yielding intermediate events is useful for query diagnostics and tracing.

[zeroxbt]

From my understanding, getClosestPeers should return only kBucketSize closest peers. This implementation currently also returns multiple "in between" peers instead coming from events other than "FINAL_PEER".
I removed this so that only final peers are returned. It could be there is a misunderstanding on my part. If that's the case, I can add it back.

[zeroxbt]

I would expect this function to yield the kBucketSize sorted peers, but it's yielding a lot more than expected (not all are FINAL_PEER), and not necessarily sorted (because it merges events yielded by both lan and wan dhts).

[achingbrain]

This feature is called disjoint paths - it's in the S/Kademlia paper and this impl used to support it.

Notably the go KAD-DHT implementation does not have this feature which is why it was removed from this repo, though I notice the rust implementation does support it so there's no harm in re-adding it here, and is certainly worth doing if it makes the queries run faster.

Thanks for opening this!

[achingbrain]

The final thing to do to get this in is to write some tests.

For an example you can see how we create predictable network topologies in test/query.spec.js and run a query which records the peers it passed through to get to the final value.

You should be able to validate in a test that queries stop when passing through the same node twice.

[achingbrain]

Thanks!

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