Switch to event-queue model to avoid spurious thread-creation

This extension works by creating 1-2 threads for every note played, one to start the note playing and one to stop it. This can lead to literally thousands of threads being created within a second or two of each other. Instead of starting a thread for each start/stop, it would be better to have a pool of x (probably 1-2) background threads which take various start/stop operations from a DelayQueue and execute them. This would reduce the total number of threads and conceptually simplify the model this extension uses.

Start by locating the extension code that creates threads for note start and stop operations, then read how those operations are scheduled and ended. Compare that flow with Java's DelayQueue; done means a small background-thread pool handles delayed start/stop work without creating a thread for every note.