store image job results indefinitely

[ssube]

Jobs are immediately removed from memory if the client comes back to check /ready, with #55, but abandoned jobs are not removed until the Executor starts to prune them, which defaults to 50.

Look for jobs that finished more than N minutes ago and remove their futures, probably as part of the /ready endpoint.

[ssube]

The FutureCollection used to store jobs does not record when they were started, making it difficult to prune jobs on a regular basis, but it does limit the overall growth.

If a job has been removed from the collection and the client comes to check on that job, they will not get an affirmative value. Adding a check for the output file might work, or keeping a database of all the image parameters and their start/end times.