Akka source creation function should return a future

[gmethvin]

It should be possible to pass a () => Future[Consumer[T]] to create the Akka source, so we can easily use the async consumer creation API and avoid blocking the Akka thread pool.

[KannarFr]

This could be related to #262, so a Future[Either[?]]?

[gmethvin]

The main issue here is with being able to use the async API, since I don't want to block the Akka thread pool creating a consumer.