[Cashtab] [WS Update Engine Part 1] Convert wallet utxo refresh interval from hardcoded number to a state variable

Summary:
T2447

Part 1 (of 4 planned) stacked diff for converting Cashtab from a static refresh rate to an on-demand refresh rate depending on Websocket transaction messages

This diff converts the hardcoded interval to a state variable.

Test Plan: npm start, receive a tx, wallet updates as before

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D11656