Fix minor blemishes discovered by findbugs.
Small documentation improvements.
Refactor some tests so that they use the jtreg timeout, which is user-configurable, instead of engineering their own.
Improve code sample and test for ExecutorCompletionService.

It's a (small) mistake that ConcurrentSkipListMap.SubMap
implements Cloneable, but doesn't implement clone. It seems no one ever noticed before.