[Cashtab] Implement latest chronik-client

Summary:
Implement updated chronik-client in Cashtab

Cashtab implemented backend switching before it was integrated in the library. Deprecate Cashtab's switching mechanism and rely on the library.

Note there is an expected behavior change in Cashtab from this implementation. Previously, Cashtab would cycle through available chronik instances perpetually until one connected. Now, once all instances fail, the app will not reconnect unless it is refreshed.

Test Plan:
npm test

npm start and send some transactions

Change the url array in config.chronik to have some bad urls, verify they autoswitch

Change wallets and send txs, verify notifications work

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15050