make ChildData constructor public for access from unit tests and such

When unit testing an application making use of PathChildrenCache, I wanted to make some PathChildrenCacheEvents to pass to my PathChildrenCacheListener-implementing class. However I found that PathChildrenCacheEvent contains ChildData, which has only a no-modifier constructor, meaning my unit test would have to be in the org.apache.curator.framework.recipies.cache package in order to perform these tests.