Observable.using should use unsafeSubscribe and enable eager disposal

[davidmoten]

See #2604 where it was discovered that Observable.using used subscribe instead of unsafeSubscribe which provoked a race condition leading to an IllegalArgumentException from the merge operator.

This PR uses unsafeSubscribe and adds an overload for using to optionally dispose of resources eagerly (just before completion or error). The use case for this is a synchronous observable where a downstream operation wants to reuse a resource (but because the observable is synchronous the resource cannot get disposed till the downstream completes).

Unit tests included.

[akarnokd]

I'd have a class that implements both Action0 and subscription to avoid creating too many objects:

static final class DisposeAction<R> extends AtomicBoolean implements Action0, Subscription {
    Action1<? super R> dispose;
    R resource;
    public DisposeAction(Action1<? super R> dispose, R resource) {
        this.dispose = dispose;
        this.resource = resource;
        lazySet(false); // StoreStore barrier
    }
    @Override
    public void call() {
         if (compareAndSet(false, true)) {
            try {
                dispose.call(resource);
            } finally {
                resource = null;
                dispose = null;
            }
         }
    }
    @Override
    public boolean isUnsubscribed() {
        return get();
    }
    public void unsubscribe() {
        call();
    }
}

[davidmoten]

Thanks @akarnokd, I've made the change and squashed commits.

[akarnokd]

Could you add @Experimental here? In addition, could you replace tabs with 4 spaces in OnSubscribeUsing?

[davidmoten]

Replaced tabs with spaces, added @experimental annotation to new overload and fixed typo in javadoc, squashed commits

[benjchristensen]

This seems reasonable to me. I don't use 'using' so trust the conversation you've had to address the necessary functionality. Reading through the other PR this does seem necessary and I agree that 'unsafeSubscribe' is appropriate for this.