codeintel: Add janitor process to api-server

Created by: efritz

This janitor will periodically ensure that a bundle file exists for every processed upload record in the database. If there is a missing bundle file, the associated db record is deleted.

The bundle manager has a symmetric janitor process that will delete any bundle files for which there is no record.

This should keep the upload record and the processed files in sync in case there is any manual intervention on the filesystem, or programming errors that create spurious files (like opening empty SQLite dbs, as we did for a short period of time on dot-com).