Always instrument ActiveSupport::Cache

The current approach is broken because it uses a thread local value which
means on multi-threaded environments it has to be turned on per thread.
Secondly, ActiveSupport::Notifications does not instrument items when there
are not subscribers so this flag is unnecessary.
We only need to improve the test to tell people we will remove these methods on future. Something like:

```
ActiveSupport::Cache.instrument is deprecated and will be removed on Rails 5. Instrumentation is now always on so you can safely stop of using it.
```