Add Purge/Maintenance Action

[tedivm]

We need to have some sort of purge or maintenance action that can be called outside of the normal website calls. This will pass through to the stash "Purge" function, which (depending on the backend drivers used) will do things like filesystem cleanup of expired items or rekeying of redis items to take into account new servers.

[jhallbachner]

This ties into a general need to provide more fine-tuned cache maintenance commands on the CLI, and reminds me of another issue I need to add.