JerseyCompletionStageRxInvoker should use AsyncInvoker

[olotenko]

Expected: configure asynchronous thread pool the size of CPU count, be able to saturate CPU with long-running requests to other services.

Actual: CPU mostly idle. RxInvoker uses SyncInvoker, and Jersey Client does the straightforward thing, which consumes the threads waiting for response.

If #793 is addressed at the API level, so much the better. If #793 is not fixed, at least we probably can assume that AbstractRxInvoker.getSyncInvoker returns the same SyncInvoker that JerseyCompletionStageRxInvoker passed to it in the constructor.

A prototype fix with such type cast demonstrates the thread utilisation can become as Expected.

[olotenko]

Apologies, meant to file as a Jersey issue. Moved to eclipse-ee4j/jersey#4255.