LSIF: Endpoints for dumps in lsif-server

Created by: efritz

Add endpoints to search and return dumps for a particular repository.

This will help us in troubleshooting whether or not a dump exists for a commit and will eventually power a UI which will show all commits for which there is intelligence on a repository. This is in a similar spirit but distinct from RFC 46.

This is somewhat necessitated after adding code that prunes old dumps.