[Cashtab] Cleanup interval from 30s to 5s

Summary:
T2624

Cashtab refreshes every time the websocket detects a new tx. Cashtab also checks on the utxo set at a fixed interval.

Right now, this is at 30s. However, since chronik triggers checks, but bch-api provides the utxos, occasionally we see instances where users must wait 30s for utxo set to refresh.

Reduce this to 5s while we work to get bch-api out.

Test Plan: npm start, send and receive txs. You may not ever see a 5s delay. If you do, note it's 5s and not 30s.

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D11885