Refactor TaskQueue to use RetryableQueue interface

[PauloMigAlmeida]

Problem
When creating a custom <Executor className="myCoolClass">, developers may also want to use their own BlockingQueue<Runnable> implementation instead of the default TaskQueue.

While ThreadPoolExecutor accepts any BlockingQueue<Runnable> in its constructor, it assumes/expects the queue is a TaskQueue when handling RejectedExecutionException, as shown in the snippet below:

tomcat/java/org/apache/tomcat/util/threads/ThreadPoolExecutor.java

Lines 1338 to 1346 in 6cfb392

	if (getQueue() instanceof TaskQueue queue) {
	// If the Executor is close to maximum pool size, concurrent
	// calls to execute() may result (due to Tomcat's use of
	// TaskQueue) in some tasks being rejected rather than queued.
	// If this happens, add them to the queue.
	if (!queue.force(command)) {
	submittedCount.decrementAndGet();
	throw new RejectedExecutionException(sm.getString("threadPoolExecutor.queueFull"));
	}

Proposed changes
Introduce a RetryableQueue<T> interface with the .force(T) method. This allows users to implement custom TaskQueue-like queues that can integrate with the existing rejection handling logic by implementing this interface.

[markt-asf]

This looks to be a reasonable request. There are a few additional changes that will also be required. I'll merge this, make the additional changes and then back-port the combination.

[PauloMigAlmeida]

Thanks @markt-asf !