Add command to clear stale cacheops keys

[browniebroke]

Building on top of the excellent work from @nicwolff in #323.

I found the code very useful and almost good for us. The main problems we had were:

• we didn't want run this in a while True loop, we'd rather run it once a day or so
• we wanted it in Celery task. While it's probably not something to be implemented in cacheops, but more in our application, having the logic in a command was limiting its reusability, so I moved it into a function
• the admin command seems quite useful hence I left it

Note that I left the first commit to credit the original author.

I'm willing to put the extra steps to get this in cacheops. Is it something that you'd accept @Suor? If yes, let me know if it's good enough or what's missing. I guess tests & documentation would be nice 😄 Before I invest too much time, are you happy with how things are called?

[Suor]

Thanks for the changes, looks good.

[browniebroke]

Hello 👋🏻

Sorry to revive this PR, I wanted to check if we can expect this to appear in a release relatively soon? If the release is around the corner, I can wait a bit more for it. On the other hand, I totally get it if you don't think you'll have time to do anytime soon or if there are other blockers, I can just install from the GitHub commit.

Thanks again for this great lib!

[Suor]

I am preparing release now, probably, make it in a couple of days.

[nicwolff]

Oh hey, I just saw this! Thanks! One more reason to update our cacheops install ツ