Let worker threads check cache instead of dispatcher.

@JakeWharton 

This effectively makes the hunter responsible for checking the cache instead of the dispatcher.

When a `get()` request is executed now the logic for fetching the bitmap is exactly the same as if it was for an `into()` or `fetch()` request. 

Previously if you did a `get()` it wouldn't check the cache in the background thread.

The old code also used to make worker threads check the cache instead of the dispacher.