[Cashtab] Sync utxo set on load

Summary:
Depends on D15049

T3384

Sync wallet immediately on load instead of waiting 10s to sync.

Test Plan:
npm test

Go to cashtab.com and send some txs with the same wallet you have loaded at localhost:3000
npm start and balance updates immediately on load.

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15151