[keyserver][lib] handleAsyncPromise -> ignorePromiseRejections

Summary:
As part of enabling the unused-promise Flow lint (ENG-5907), I want to make two changes to this function:

1. Renaming it to ignorePromiseRejections. I think this captures what it does much better.
2. Move it to lib. The Lint exposed some issues in lib utilities that we use from keyserver, and to fix those we'll want access to ignorePromiseRejections from lib.

Test Plan: Flow

Reviewers: michal

Reviewed By: michal

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10158