bitbucket: Switch to rate limit registry

Created by: ryanslade

We'll now explicitly check rate limiting at different layer, specifically when running background changeset and permission syncers. We still keep the old failsafe rate limiter in place because Bitbucket doesn't provide rate limiting on their side.

This change makes the way rate limiting works across different code hosts more consistent.

Part of: https://github.com/sourcegraph/sourcegraph/issues/8546