[2.3.x] Update javadocs.
[2.3.x] Changes for https://java.net/jira/browse/GRIZZLY-1543 (Future.get() may return null).
+ [2.3.x] Changes for https://java.net/jira/browse/GRIZZLY-1543 (Future.get() may return null).
remove FutureTask's code (related to callable) we don't need.