* Special means of indicating that an executable will proceed in the background without consuming a native thread ({@link Executor}).
* May be thrown from {@link Executable#run} after doing any preparatory work synchronously.
* <p>{@link Executor#isActive} will remain true (even though {@link Executor#isAlive} is not) until {@link #completed} is called.
* The thrower will need to hold on to a reference to this instance as a handle to call {@link #completed}.
* The thrower could hold on to a reference to this instance as a handle to call {@link #completed},
* or look it up later via {@link Executor#getAsynchronousExecution}.
* <p>The execution may not extend into another Jenkins session; if you wish to model a long-running execution, you must schedule a new task after restart.
* This class is not serializable anyway.
* <p>Mainly intended for use with {@link OneOffExecutor} (from a {@link FlyweightTask}), of which there could be many,
