[lib] Wrap spinPromises in ignorePromiseRejections

Summary:
The unused-promise Flow lint revealed an issue here. We use SequentialPromiseResolver in keyserver, where an unhandled promise rejection could crash the whole server. But we aren't doing anything to handle a potential promise rejection here.

This diff wraps spinPromises in ignorePromiseRejections. This way, if the promise rejects we will print an error instead of crashing the whole keyserver.

Depends on D10160

Test Plan: Flow

Reviewers: michal

Reviewed By: michal

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10161