CURATOR-124 - PathChildrenCache StartMode doc

Updated PathChildrenCache.StartMode.NORMAL JavaDoc to
accurately depict the cache startup behavior.

Added test testChildrenInitializedNormal to assert
the startup behavior indicated in the JavaDoc.
Updated per review comments

Updated BUILD_INITIAL_CACHE doc to indicate that
the start() method with this mode will cause
a synchronous rebuilding of the cache.

Removed extra line.
Merge remote-tracking branch 'temp/CURATOR-124' into CURATOR-124