Convert into the Electrum SPV proof format from the bitcoind merkleblock format

[shesek]

bwt currently constructs the electrum spv proofs for transactions using the list of txids in the confirming block (retrieved via getblock with verbose=1), which is a rather large list that needs to be fetched separately for every transaction.

This could be improved by converting the bitcoind merkleblock spv proof into the format expected by electrum, which is implemented by EPS here.

Ideally though, users should probably just configure electrum with --skipmerklecheck and bwt with --electrum-skip-merkle, as the spv proofs don't really add much in the context of a trusted self-hosted full node setup.