Ensure we don't confuse the double hash of a transaction with it's txid

[ffranr]

In some instances we've confused the double hash of a transaction with the transaction ID (txid). I think this is one example:

taproot-assets/rpcserver.go

Line 1383 in e6b0516

AnchorTxid: p.AnchorTx.TxHash(),

There may be others.

This confuses users because they can't use the "txid" as to look up a transaction in a mempool explorer. I think we should check our other uses of transaction hash and transaction ID.

I've created this PR in btcd to help make our code clearer: btcsuite/btcd#2055

[dstadulis]

itest might be the location for the text to be added

[ffranr]

If we are to use string for txid then we should probably rename SQL field chain_txns.txid to tx_hash. This field is a blob and we currently set it to the tx hash.

[ffranr]

With the exception of the db change I mention in this comment, this issue will be redundant once we complete #654 .