[Cashtab] Set ConnectionStrategy.ClosestFirst for initialization

Summary: Set ConnectionStrategy.ClosestFirst for initialization chronik-client will start asynchronously and others will wait.

Test Plan: npm test and test it manually in the local demo

Reviewers: Fabien, bytesofman, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: bytesofman, O1 Bitcoin ABC, #bitcoin_abc

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D18041