@Override
                        public void completed() {
                            final AsyncExecCallback callback2 = callbackRef.getAndSet(null);
                            if (callback2 != null) {
                                callback2.completed();
                            } else {
                                asyncExecCallback.completed();
                            }
                        }
Request executor in the request execution chain that is responsible for
transparent client-side caching.
<p>
The current implementation is conditionally
compliant with HTTP/1.1 (meaning all the MUST and MUST NOTs are obeyed),
although quite a lot, though not all, of the SHOULDs and SHOULD NOTs
are obeyed too.

@since 5.0
httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/AsyncCachingExec.java