CURATOR-157 Avoid stack traces on close

Avoid error-level stack traces closing NodeCache followed by closing CuratorFramework
code style: curly braces on new line
CURATOR-157 early return from refresh/processChildren when being closed

The earlier change of CURATOR-157 let the close() method wait until a background refresh was finished. This change lets the refresh quit early when it sees the cache is closed, this could be important for large caches.