@Override
            public void completed() {
                final AsyncExecCallback callback1 = callbackRef.getAndSet(null);
                if (callback1 != null) {
                    callback1.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