R1:5903ed650c20

Move completed tasks to an "archive" table and delete them in the GC

Summary:
Currently, when taskmasters complete a task it is immediately deleted. This prevents us from doing some general things, like:

- Supporting the idea of permanent failure (e.g., after N failures just stop trying).
- Showing the user how fast taskmasters are completing tasks.
- Showing the user how long tasks took to complete.

Having better visibility into this is important to Drydock, which builds on the task system. Also…