Add electrum-backed search for close-by coinjoin transactions missed by Dumplings threshold

[petrs]

The detection by Dumplings is limited by threshold value for number of inputs (we use 20 atm). Smaller transactions are therefore not collected and missed (otherwise there would be too many false positives). This is not a problem for large coordinators (zksnacks, kruw), but is for small or starting ones.

For WW2 with high number of remixes, we can detect additional coinjoin txs simply by looking incoming/outgoing transactions referenced by multiple inputs/outputs of a known coinjoin transaction, followed by subsequent check WW2 detection rules (repetition of standard denominations etc.) to distinguish between fresh input/output and close-by coinjoin.

Requires Electrum-like backend for efficient analysis of outputs.