[Cashtab] Prepare for migration to psf bch-api with new utxo key
PSF has upgraded bch-api to add features that will support Cashtab (BigNumber.js for SLPA, rate limits in env variables). Cashtab will move to the PSF version of bch-api (away from custom fork) for better support and standard repo.
The updated bch-api removes duplicate key entries. The API used to return both value: and satoshis: keys in utxo objects. These were identical. Cashtab used the satoshis: key. However, the upgraded api returns only value:.
This PR switches to value: instead of satoshis: in preparation for upgrading the backend, which will return only value:
Observe balances load as before
Observe sending BCHA and SLPA both work as before
Reviewers: alcipir, majcosta, Fabien, #bitcoin_abc
Reviewed By: Fabien, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D8885