PersistenceApi#fromFuture (or similar)

[dilation]

If possible, we should create a method on PersistenceApi like this:

def fromFuture[A](f: Future[A], sync: Boolean = false): Proc[A]

The hard part seems to be, that when sync is true, no messages should be delivered until the Future finishes. (Also, maybe we'd need a timeout too, in case the Future never finishes.)

The use case for this is described here: https://gitter.im/nokia/akka-typed-persistence?at=5980c287f5b3458e30a0138e