Add ability to invalidate (delete) transient data.

[CodeProKid]

We should add the ability to delete or invalidate a transient so that we force it to regenerate. I'm not 100% sure on what the implementation should look like for this, but I think it would be nice to essentially "invalidate" it, so that we use the built in mechanism's to asynchronously regenerate the data for it. This will need some thought.