Add maintenance routine to periodically prune the job index DB

[shonfeder]

Originating from this discussion #293 (comment)

@mtelvers noted the following

• The index database is never pruned.
• As the tables grow, perhaps an expensive query begins to eat up a lot of time.
• If the time for a query exceeds the configured timeout, this may cause retries to keep executing the query

Whether or not this was the cause of service issues that motivated the discussion, I think we all agree it would help to periodically prune old data from the DB, once it is beyond the point of being useful.