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

I'm just concerned it's a larger problem. Executors are used throughout Curator and I've relied on shutdownNow to work correctly. I didn't know about the bug. So, I'd feel better having an open issue to deal with it directly. Curator already has the CloseableExecutorService wrapper and my gut feeling is that it can be fixed inside of there.