Set up a cache cleaning service for EFS

[alexwlchan]

Related to #655.

Once we have Loris caching masters to EFS, we need to keep the cache relatively tidy to avoid running up big costs. We should create a service that cleans our EFS mounts for us. These scripts from the Loris repo are probably a good start:

https://github.com/loris-imageserver/loris/blob/development/bin/loris-cache_clean.sh
https://github.com/loris-imageserver/loris/blob/development/bin/loris-http_cache_clean.sh

[alexwlchan]

We’re going to pair on this tomorrow.

[alexwlchan]

Still to do on this: set up a scheduled task, which is blocked on hashicorp/terraform-provider-aws#977 being released.

[alexwlchan]

(Terraform 1.10 is in RC1, so I assume this is imminent – I don’t think it’s worth writing this as a Lambda just to throw it away days later.)

[kenoir]

Have manually added this as a scheduled task. We'll remove them when the next version of terraform is released.