CURATOR-166: Allow ZKPaths to build paths with an arbitrary number of children

ZKPaths' makePath currently only accepts one parent and one child nodes.
This changes makePath to have a varargs parameter so it can accept more than one child.
Yes, that makes sense. I updated the change. In order to avoid code duplication and multiple StringBuilder instantiations, I've added a new auxiliary method that mutates a StringBuilder passd as an argument.