refactoring class-hash to be ractor-safe

mutable constants can't be shared across ractors; this changes that design to define the required variables as constants on the Resource class, which makes them reachable using ractors; the ClassHash is kept in order not to break integrations relying on its existence, but under the hood it's doing the same thing

lib/resolv.rb

@@ -1699,7 +1699,14 @@ class Resource < Query
 
       attr_reader :ttl
 
-      ClassHash = {} # :nodoc:
+      ClassHash = Module.new do
+        module_function
+
+        def []=(type_class_value, klass)
+          type_value, class_value = type_class_value
+          Resource.const_set(:"Type#{type_value}_Class#{class_value}", klass)
+        end
+      end
 
       def encode_rdata(msg) # :nodoc:
         raise NotImplementedError.new
@@ -1737,8 +1744,10 @@ def hash # :nodoc:
       end
 
       def self.get_class(type_value, class_value) # :nodoc:
-        return ClassHash[[type_value, class_value]] ||
-               Generic.create(type_value, class_value)
+        cached = const_defined?(:"Type#{type_value}_Class#{class_value}") &&
+                 const_get(:"Type#{type_value}_Class#{class_value}")
+
+        return cached || Generic.create(type_value, class_value)
       end
 
       ##
@@ -2907,4 +2916,3 @@ def DefaultResolver.replace_resolvers new_resolvers
   AddressRegex = /(?:#{IPv4::Regex})|(?:#{IPv6::Regex})/
 
 end
-