Creates a function that caches results of calls to app.injector.instanceOf[T]. The cache speeds up calls when called with the same Application each time, which is a big benefit in production. It still works properly if called with a different Application each time, such as when running unit tests, but it will run more slowly.
Since values are cached, it's important that this is only used for singleton values.
This method avoids synchronization so it's possible that the injector might be called more than once for a single instance if this method is called from different threads at the same time.
The cache uses a SoftReference to both the Application and the returned instance so it will not cause memory leaks. Unlike WeakHashMap it doesn't use a ReferenceQueue, so values will still be cleaned even if the ReferenceQueue is never activated.