Specialized delay queue. To mesh with TPE declarations, this class must be declared as a BlockingQueue!(Runnable) even though it can only hold RunnableScheduledFutures.
A {@link ThreadPoolExecutor} that can additionally schedule commands to run after a given delay, or to execute periodically. This class is preferable to {@link java.util.Timer} when multiple worker threads are needed, or when the additional flexibility or capabilities of {@link ThreadPoolExecutor} (which this class extends) are required.