change Ruby.getConstantInvalidator() to take a constant name
switch over some obvious things to use getConstantInvalidator(String)
fix up calls to invalidateConstantCache() in RubyModule
teach RuntimeCache about invalidator per constant name
use ConcurrentHashMap instead of Hashtable
use CHM.putIfAbsent instead of explicit synchronization
remove volatile from invalidator declaration
don't synchronize on invalidator getter methods