Review Java callbacks and threading

[jadamcrain]

The CompletionStage returned by future methods will continue execution on a Tokio thread if any of the async combinators are used, e.g. CompletionStage.theAccept(...).

This can cause problems if the chained computation calls anything that might use block_on.

Java doesn't have a global executor thread poll like C# does. We need to do one of the following:

1. Carefully review our usage of block_on in each library (or remove it altogether).
2. Initialize a static thread poll on which the Java backend can complete futures.

[jadamcrain]

This isn't a panacea, but the various combinators allow you to specify an executor on which to resume:

https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CompletionStage.html#thenApplyAsync-java.util.function.Function-java.util.concurrent.Executor-