Rework the neighboursAtDistances to be more suited for several distances

[kdeme]

See comment here: https://github.com/status-im/nim-eth/pull/295/files#diff-99f5b2699612c1f155df8cbc305892e4R327

It was already not very well suited because of the "at distance", see: https://github.com/status-im/nim-eth/pull/295/files#diff-99f5b2699612c1f155df8cbc305892e4R333

But now for multiple distances, depending on how many we might and can return, it could be beneficial if we actually search for those specific distances instead of grabbing all neighbours and then checking what we got.