Add listaddressgroupings RPC method from Bitcoin Core

[JeremyRand]

It would be useful to add Bitcoin Core's listaddressgroupings RPC call to Electrum. This would improve the ability of users to evaluate their anonymity.

[ecdsa]

A pull request would be welcome.
I think the coin chooser could use this.

[JeremyRand]

One interesting subtlety of how listaddressgroupings applies to Electrum is that Electrum contains some optional features that try to obfuscate the change address; AFAIK Bitcoin Core doesn't have any such functionality. So it might be useful to have an optional parameter in Electrum's implementation that controls to what extent obfuscated change addresses are considered to have been "made public". (This isn't a requirement, just a "nice to have" bonus feature.)

[SomberNight]

So this would just group addresses by "Common Input ownership Heuristic"?

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Consider the following scenario:
Addresses A and D are in wallet. Addresses B and C are external (not in wallet) addresses.
There is a transaction T1 with inputs from A and B.
There is another transaction T2 with inputs from B and C.
Another transaction T3 spends from C and D.

Do you now want to group A and D together? (I assume yes; just wanted to point out it might not be trivial to implement. Note that transaction T2 is unrelated to the wallet.)

[JeremyRand]

Do you now want to group A and D together?

Hmm, what does Bitcoin Core do in this case? If Bitcoin Core doesn't group A and D, then I'd be fine with having Electrum not do so either (although maybe in that case a bug should be filed against Bitcoin Core).

[JeremyRand]

Do you now want to group A and D together?

Relatedly, one intended use case for listaddressgroupings is to figure out what information to hide from the server, so it's probably a bad thing for this method to produce network traffic to the server. Which means downloading transactions that aren't already part of the wallet is probably not doable unless it's done very very carefully (e.g. over a separate server TCP connection and over a separate Tor circuit).