[Cashtab] [Cashtab] [final bch-api deprecation p3] Initialize ChronikClient as a state variable

Summary:
T2730, T2735

Depends on D12788

Move ChronikClient to a state variable

Test Plan:
npm start
In dev log, confirm chronik still subscribes to websockets as expected

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12789