[chronik mock] Make sure history function is async so Promise.all() tests work

Summary:
Discovered this bug in creating D13707

The chronik function to get one page of tx history is async. For efficiency, getting multiple pages is usually done with Promise.all(). This will only work on an array of promises, which need to be async functions.

Test Plan:
See passing unit tests in D13707
Check unit tests for alias-server and ecash-herald here

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13727