Visible at tip calculation is racy

Created by: efritz

When enabling more than one worker (via https://github.com/sourcegraph/sourcegraph/pull/10912, but also with horizontal scaling of worker pods), the visibility calculation for LSIF dumps is racy. This will likely be solved by implementing RFC 186, which changes the way nearest queries are calculated.

This issue is used to track the racy behavior to ensure it doesn't exist after updating the implementation.