[ecash-agora] ts linting in agora.ts

Summary:
typescript does not like while (true). Apparently this is the "right way" to "do something forever"

https://eslint.org/docs/latest/rules/no-constant-condition

use const instead of let for a constant

Test Plan: npm test and CI

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D16928