[keyserver] Don't block innerPerformAuth return on sleep

Summary:
In an earlier version of this stack, in innerPerformRecovery I would fall back on legacy login, and without this change the fallback would have to wait on this sleep.

This didn't used to matter before because nothing actually waits for innerPerformAuth to complete. And since I eventually came to the conclusion that the fallback was a bad idea (we'd be sending user passwords to keyservers), it actually still doesn't matter.

I decided to put this diff up anyways because I think it's probably better for us to avoid blocking the return of innerPerformAuth on this sleep.

Depends on D11500

Test Plan: Confirm in the earlier version of the stack that when falling back on legacy login, there was no delay due to this sleep

Reviewers: tomek, inka

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D11501