Add exception handling methods to CompletionStage and CompletableFuture

The CompletionStage interface and CompletableFuture implementation lack some methods for dealing with exceptional outcomes that users need and expect based on presence of related non-exception-handling methods. Requests for these have been posted on concurrency-interest list; for example: http://cs.oswego.edu/pipermail/concurrency-interest/2018-September/016512.html

Addressing these requires methods to cover all variants of exceptionally{Compose}{Async}:

 exceptionally(Function<Throwable, ? extends T> f) // already exists
 exceptionallyAsync(Function<Throwable, ? extends T> f);
 exceptionallyAsync(Function<Throwable, ? extends T> f, Executor e);
 exceptionallyCompose(Function<Throwable, ? extends CompletionStage<T>> f);
 exceptionallyComposeAsync(Function<Throwable, ? extends
CompletionStage<T>> f);
 exceptionallyComposeAsync(Function<Throwable, ? extends
CompletionStage<T>> f, Executor e);

Default implementations in CompletionStage have tolerable overhead; direct implementations in CompletableFuture are more efficient.

We currently have these, as well as tck tests in jsr166 repo.