This is the first step toward caching bcha utxos, slpa utxos, and tx history in local storage.
Currently the largest bottleneck on the server is that the web wallet must poll the server for full utxo history on startup. If state is saved to local storage, then Cashtab can instead only poll for utxo changes, and then only poll changed utxos for SLPA status.
This change does not add utxos or txhistory to state. It only adds what is already stored in state to the cache. Future diffs will add info required for caching, check for state in storage before polling the server, and reduce server calls from all utxos on every startup to only required calls for uncached utxo data.