Limit disk space

[ferchault]

If disk space is exhausted, hmq is not operational. It should handle this gracefully:

• define limit for "functions" and "tasks" collection in GB as environment variable
• upon function insert or result insert or dequeue, fail if limit is reached
• dashboard: show current usage
• make sure collections are autocompacted
• implement CLI "hmq archive" and "hmq archive --delete-after" which downloads all tag data and delete the downloaded tasks afterwards if the flag is given.