codeintel: Add a queue for dependency indexing

Created by: efritz

Replace the QueueIndexesForPackage call from the worker. Instead, the worker will create a new queued job record in a new queue processed by a worker in the frontend (we may move this in the near future). These new records are attached directly to an lsif_upload record so do not need to be cleaned periodically like other records may (deletion of one is followed by the other).

Fixes https://github.com/sourcegraph/sourcegraph/issues/20838.

Future tasks:

• Add dashboards (some metrics already being emitted, should add additional) https://github.com/sourcegraph/sourcegraph/issues/21606
• Add a way to de-hard-code the list of accepted repository IDs (a simple db table is a good start) https://github.com/sourcegraph/sourcegraph/issues/21607
• Add a way to re-trigger dependency scanning for a particular index https://github.com/sourcegraph/sourcegraph/issues/21608
• Add way to show job progress in the UI (with links to spawned index jobs) https://github.com/sourcegraph/sourcegraph/issues/21609
• Perform ReferencesForUpload in batches and log progress https://github.com/sourcegraph/sourcegraph/issues/21610
• Perform QueueIndexesForPackage transactionally https://github.com/sourcegraph/sourcegraph/issues/21611