Make Client's Request and Response abort() fully async

[lorban]

The client's Request and Response interfaces contain the following method:

boolean abort(Throwable cause);

The contract of that abort() method is to asynchronously notify a running request that it has to be aborted.

That abort() method is meant to run concurrently with the servicing of the request, meaning that the request may be aborted, or it may finish before the abort takes effect, and some actions must happen depending on the outcome of this race condition: either abort() won the race and -for instance in H2- a reset needs to be sent, or abort() lost the race and it basically must be treated as a no-op.

The problem of the current method signature is that it returns a boolean that is returned immediately while that boolean is supposed to be computed during the async execution. This has hard implications on how abort() can be implemented, and prevents some refactorings of the client's internals.

The solution is to change the contract such as the boolean is delivered asynchronously:

CompletableFuture<Boolean> abort(Throwable cause);

[gregw]

Can you provide a description for the PR, perhaps suggesting which of the 40 files contains the core of the changes.