Improve DFCompositeImageManager dispatch logic for preheating requests

This is how it works right now:

```
- (void)startPreheatingImagesForRequests:(nonnull NSArray *)requests {
    for (DFImageRequest *request in requests) {
        [DFManagerForRequest(request) startPreheatingImagesForRequests:@[request]];
    }
}
```

Composite image manager should use a dispatch table instead.
