[keyserver] Ignore potential to crash on promise rejections in keyserver preamble

Summary:
We use an async IIFE in lieu of top-level await (which Flow doesn't support) to initialize the keyserver.

In most cases we don't want to crash on promise rejections in keyserver. But if we encounter an error in the preamble, we should probably crash, since it indicates that the keyserver is not able to initialize itself.

This diff doesn't change any behavior. We're just using the void keyword to silence the unused-promise Lint, which is recommended by the Flow team. See the "Fire-and-forget" part of this blog post.

Depends on D10178

Test Plan: Flow

Reviewers: michal

Reviewed By: michal

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10179