[keyserver] Use async version of brotliCompress

Summary:
It's generally better for performance to let this be async, especially if it's implemented natively and can take advantage of threading / parallelism.

Putting this as a separate diff because I think it has some separate risks.

Depends on D9294

Test Plan: See parent diff's test plan

Reviewers: atul, michal, kamil

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D9298