Make the Future.join tryComplete function usable outside of the Future context

[danieldietrich]

We removed Promise in favor of Future.join(...) and made the tryComplete function accessible in a managed context only.

We have an example that might show the necessity of making the tryComplete function accessible from the outside (like Promise did before): Concurrent recursive function memoization.

The current API:

Future.join(tryComplete -> {
    final boolean isCompleted;
    if (ok()) {
         isCompleted = tryComplete.test(Try.success("ok"));
    } else {
         isCompleted = tryComplete.test(Try.failure(new Error()));
    }
    logger.log(isCompleted);
}

Current possibility to externalize tryComplete:

AtomicReference<CheckedConsumer<Predicate<Try<? extends T>>>> promise = ...;
Future<?> future = Future.join(tryComplete -> promise.set(tryComplete));

Now we take a look, if this is sufficient to re-implement #2096 (e.g. using a data structure instead of AtomicReference, it was only an example).

The solution to add more flexibility is that tryComplete returns the self reference to the underlying Future instead of a boolean complete status.

[danieldietrich]

Our Future implementation will be dropped. There will be no Vavr 1.0 module vavr-concurrent.

Reason: Implementing our own concurrency library is too error-prone, concurrency is a hard problem. There are many(!) alternatives, first of all Java's native CompletableFuture