LSIF: Move update-tips functionality into convert job

Created by: efritz

We track which LSIF dumps are 'visible' from the tip of the default branch so that we can return the most relevant results in global find references operations. Previously, the lsif-server allowed uploads for commits which were not known to gitserver. This means that it was possible to upload an LSIF dump for a commit after the commit exists but before the gitserver was synced with the code host. Since then we've added a ResolveRev call that will ensure that, on upload, gitserver is up to date with the code host.

In order to support this ordering, we would update which commits were visible to the tip of the default branch on a scheduled job, running for every repository every five minutes. This is now overkill, as we can determine which dumps are visible from the tip at the time of an upload for a repository and this data does not change independently of LSIF dump uploads (no LSIF dump will "become visible" from simply adding commits to the default branch).

This PR simplifies this process:

• removes the update-tips job
• invokes {discover,update}{commit,tip} from convert job
• simplifies {discover,update}{commit,tip} methods in xrepo (discoverCommits now returns a map instead of a flattened array for easier merging)
• small refactor of convert job