Conditionally wrap Thread.sleep(..) call in collectResults(..).

Replace Thread.sleep(..) with Future.get(timeout, :TimeUnit) call for 1 millisecond. As such, the Future.isDone() (and Future.isCancelled()) calls are no longer necessary. Simply try to get the results, and if a TimeoutException is thrown, then simply set done to false.

Add test coverage for ClusterCommandExecutor collectResults(..) method.

Cleanup compiler warnings in ClusterCommandExecutorUnitTests.

Closes spring-projects#2518

Author: jxblum