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

Keep Thread.sleep(..) to avoid the busy (hot) loop when the Future executions are not yet complete.

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

Cleanup compiler warnings in ClusterCommandExecutorUnitTests.

Reduce Thread.sleep(1L) to 1 millisecond.

Closes spring-projects#2518

Author: jxblum