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

Summary:
Node.js runtimes such as keyserver can crash on unhandled promise rejections. In general, that's something we probably want to avoid.

However, in the context of scripts, we actually want an unhandled promise rejection to crash the script. In this diff, we add void declarations to silence the unused-promise Lint, and otherwise leave the behavior as-is.

Regarding the void keyword: the Flow team recommends using the void keyword to silence the unused-promise Lint. See the "Fire-and-forget" part of this blog post.

Depends on D10176

Test Plan: Flow

Reviewers: michal, atul

Reviewed By: atul

Subscribers: atul, tomek

Differential Revision: https://phab.comm.dev/D10177