The URI won't have a dot com in it, a reasonable value for tests would be something like "file://tmp/ivarator" or "hdfs://tmp/ivarator/"

It looks like cardinality threshold stuff was reverted as part of this work, please correct this

Let's follow a "get or create" pattern. This allows lazy initialization of the ivarator config. This means a query that does not require an ivarator can avoid some extra object initialization (and later, some potentially expensive I/O calls to the configured filesystem).

This would become

getIvaratorConfig().setMaxIvaratorResults(maxIvaratorResults)
..
return getIvaratorConfig().getMaxIvaratorResults()
..
public IvaratorConfig getIvaratorConfig(){
  if (ivaratorConfig == null) {
    // create a fresh config with default values
    ivaratorConfig = new IvaratorConfig();
  }
  return ivaratorConfig;
}

This looks great! Add a class level comment that this class encapsulates all ivarator configs in a single object to simplify serialization of configs between the webservice and tablet servers