Flask requires Asyncio support for Dataloaders

[wakemaster39]

With the death of promises, https://github.com/syrusakbary/aiodataloader is the current only implementation of dataloaders that I can find. To use this in a synchronous framework like flask, you need to have async support. I agree the default needs for this to be off, but there needs to be a way to turn this on so that people can shoot themselves in the foot if they do it wrong.