CURATOR-157 Avoid stack traces on close

Avoid error-level stack traces closing PathChildrenCache followed by closing CuratorFramework

See details in jira: https://issues.apache.org/jira/browse/CURATOR-157
I'm still not sure I follow the logic, sorry. What is the benefit of making the close method wait before returning? It seems like you would need to acquire the lock earlier in the method to have an impact on control flow.