unify space collection attribute naming

currently space is still acting as a sort of memory manager.
For proper linking, all objects must be reachable from space, hence the plural versions like messages and addresses (even they are instances, it is the list that is important)
To dish out instance to use, the head must be kept, ie next_XXX for intergers, return addresses and messages

lib/mom/instruction/message_setup.rb

@@ -60,7 +60,7 @@ module Mom
     def build_message_data( builder )
       builder.build do
         space << Parfait.object_space
-        next_message << space[:first_message]
+        next_message << space[:next_message]
         message[:next_message] << next_message
         next_message[:caller] << message
 
@@ -79,7 +79,7 @@ module Mom
         #FIXME in a multithreaded future this should be done soon after getting
         #   the first_message, using lock free compare and swap. Now saving regs
         next_message << next_message[:next_message]
-        space[:first_message] << next_message
+        space[:next_message] << next_message
       end
     end
   end