Rewrite concat operation to not block on subscribe

The concat operator previously blocked on calling subscribe until all the
sequences had finished. In quite some cases this results in unwanted (and
unexpected) behaviour, such as when prefixing an infinite Observable
with a fixed one, for example when using startWith (which calls concat):
someInputStream.startsWith(123).subscribe(x -> print(x));
This statement will block indefinitely if the input stream is infinite. Also
on finite sequences it seems silly to have to wait for them to finish, which
seems not to be the spirit of Rx.

In this new approach the incoming observables are put into a queue, instead
of waiting for the whole sequence to finish. When the first observable
completes, the next one is taken from the queue and subscribed to, and so
on. The queue can be extended while processing the observables, and
onCompleted is only called when both the source of observables has completed
and all observables in the queue have been read.