CURATOR-126: Fix race condition in CuratorFrameworkImpl.close()

Fair point, I'll revert.

BTW: I tried tracing back through the code and was not able to locate the code path from ExecutorService.shutdownNow() -> FutureTask.cancel() under 1.6.  So I might actually be confused and thinking of a Guava executor and not a JDK one, it's possible the JDK shutdownNow() is actually safe.  If so, apologizes for the mis-info.

Either way, CloseableExecutorService.close() probably does have the issue since it does an explicit cancel(true).