<p>A callback abstraction that handles completed/failed events of asynchronous operations.</p>
@param <T> the type of the context object
<p>Callback invoked when the operation fails.</p>
<p>Callback invoked when the operation completes.</p>
https://www.eclipse.org/jetty/javadoc/9.4.7.v20170914/org/eclipse/jetty/util/Promise.html
See Implementation
<p>A callback abstraction that handles completed/failed events of asynchronous operations.</p>
@param <T> the type of the context object