Fix uncancellable reactive-streams StreamSubscriber

[i-surkov]

Fixes #3260.

Due to the changes in cats-effect async cancellation semantics, StreamSubscriber's dequeue1 became uncancellable, this PR fixes that.

I've also added identical test cases for both reactive-streams and flow interops which checks that the streams provided by StreamSubscriber is indeed interruptible (it failed for reactive-streams before the fix), and that it does call cancel() on the Subscription when interrupted.

[djspiewak]

Is the cleanup of the subscription handled around this effect? Producing Some(F.unit) basically says that there is no cleanup action which must be taken on the state resulting from nextState, and it actually corresponds to a memory leak unless we have a higher level guarantee of cleanup.

[i-surkov]

The cleanup happens with the onFinalize call in the Streams.bracket here:

def stream(subscribe: F[Unit])(implicit ev: ApplicativeError[F, Throwable]): Stream[F, A] =
      Stream.bracket(subscribe)(_ => onFinalize) >> Stream
        .eval(dequeue1)
        .repeat
        .rethrow
        .unNoneTerminate
        .unchunks

which is the only place where dequeue1 is used internally.

Although technically I guess someone could call StreamSubscriber#sub.dequeue1 directly, since it is exposed here (unlike analogous flow interop).

[i-surkov]

Hi @armanbilge, just checking if you will have some time to review this