Implement playEnumeratorToChunkerator without promises

[sullivan-]

The current implementation of this method is a little lame, using promises to communicate between the producer and the consumer. There's got to be a better approach. I'd like this method to work without any dependencies besides Play and Scala standard library, so that's a limiting factor.

[sullivan-]

On a related note, the producer Iteratee could benefit from chunking approach. I won't raise this into a separate issue, because if we do get rid of the promises, then the current non-chunking code will have changed dramatically.

But it's worth noting that a non-promise based implementation would be better if it also did chunking.

[sullivan-]

I'd like to mention that I myself am only using this adapter in a test suite, so getting a better implementation here may well be a lot more important to you than it is to me. Of course, I want to see this project beautiful and complete. But I have a lot of priorities to balance. If you care, please consider chipping in! Thank you.