Add MoreStream utility methods that accept future suppliers

[msg430]

Currently the non-deprecated versions of inCompletionOrder and blockingStreamWithParallelism require a an executor to run each transformation on, which means that that transformation will be a blocking call. This is not ideal when the actual call is itself async (i.e. hitting a method which returns a ListenableFuture already). This PR adds versions of those methods that require a future supplier, that is a method that transforms the keys into the listenable futures of the result. This method will be called by the thread that is pulling from the Stream.

This is similar to earlier problematic methods that accepted an input a stream of futures. Those were deprecated because they did not enforce actual max parallelism. That is, the underlying stream supplier may kick off more futures than the parallelism intended to allow. I believe this solution avoids those issues because we only create new futures from within the buffering spliterator, which enforces the max parallelism.

[palantirtech]

Thanks for your interest in palantir/streams, @msg430! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

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Add MoreStream utility methods that accept future suppliers

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