Refactor noise processing

[danielhuang]

The current implementation (dropping samples and introducing a delay) tends to introduce a delay which doesn't get reduced even after CPU load is reduced and processing is able to keep up again. It also ends up causing unrelated processing (e.g. EasyEffects output processing) to stutter when this happens.

Instead of keeping track of a delay, the processing thread will read samples from a ring buffer, and samples only get dropped when the ring buffer gets full.

Also fixes a resource (background thread continues to poll for samples) leak.

[danielhuang]

Mostly done; the calling thread for the plugin no longer blocks (EasyEffects expects the plugin run method to not take too long, otherwise other audio streams could stutter), and CPU load will no longer lead to audio stutters. Since there's no more spin loops, the processing thread will remain idle when there's no activity. Latency is also lower when there's no other CPU activity.

Code should be ready to use, just need some more testing.