Cache key not dependent on callback args and vice versa

[roborourke]

Having the cache key at least internally not factor in the $callback_args means this could behave unexpectedly when different callback args are passed, but the same cache key is provided.

I think this should be avoided as it's too easy to make a mistake and create difficult to debug problems.

[svandragt]

I've seen the problem in action and agree, initial implementations hashed the args into the cache_key but it's much more convenient for that to be done internally instead of in the implementation.