Support async reloader function

[renatoathaydes]

Caches are likely to reload entries using IO, so it would be nice if you could add support for async reloader functions in the Cache class.

All that's needed, I think, is to change this typedef:

typedef V LoaderFunc<K, V>(K key, V oldValue);

To this:

typedef FutureOr<V> LoaderFunc<K, V>(K key, V oldValue);

And then, fix the couple of locations invoking this function to allow for async behaviour (which seems pretty easy).

This would not be a breaking change due to Dart's great FutureOr type allowing functions to be either sync or not transparently.

[platelk]

done on 0.4.0