[Cashtab] Prepare for migration to psf bch-api with new utxo key

Summary:
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:

Test Plan:

npm start

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