[Cashtab] bug fix in utxo diff check

Summary: Wallet should compare utxo set from api to utxo set from storage, now that this is in state, and not against previousUtxos, which is the legacy way of doing it. Maintain support for unmigrated wallets.

Test Plan: npm test

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9459