Update transaction.id_from_pos response for API compliance

[jharveyb]

While building a basic electrum api CLI and testing against my personal electrs node, I noticed an issue with blockchain.transaction.id_from_pos:

/target/debug/electrum-cli ssl://$SERVER_URL get-txid-from-pos 932452 1
Error: Made one or multiple attempts, all errored:
	- missing field `tx_hash`

I'm using the rust-electrum-client crate, which expects the JSON key for the TXID to be tx_hash, not tx_id as it is now:

https://docs.rs/electrum-client/latest/electrum_client/struct.TxidFromPosRes.html

So the deserialization of the response fails. According to the Electrum spec, the key should be tx_hash, not tx_id:

https://electrumx.readthedocs.io/en/latest/protocol-methods.html#blockchain-transaction-id-from-pos

So the electrs implementation is the issue here IMO.

No test added since AFAICT this API isn't exposed via the electrum CLI tool, and I didn't see another suitable place to add one.

Great project btw - excited for the bindex work to get merged!

[romanz]

Thanks!