Implement paste expiration

[iliafrenkel]

The idea is to create a scheduler that will run on a regular interval (or maybe multiple schedulers and multiple intervals) and execute certain tasks:

• delete expired tasks
• cleanup old pastes, the ones that haven't been accessed for a long time
• refresh/invalidate the cache
• write in-memory data to file/database (such as stats)

In addition to that, the API must not show the pastes that have expired already.

Other ideas are more than welcome!

[davidnewhall]

You'll probably have to write two custom methods for this. One for DB and one for memory. Overall, it's pretty easy to do both. There's an example here, but it's very specific to the new storage interface.