Revert rev 27729.

Changing to pickle protocol 2 has severe backward compatibility
implications; changes to ZODB should not be made from the Zope
trunk regardless.

lib/python/ZEO/zrpc/marshal.py

@@ -25,7 +25,7 @@ class Marshaller:
     def encode(self, msgid, flags, name, args):
         """Returns an encoded message"""
         # (We used to have a global pickler, but that's not thread-safe. :-( )
-        pickler = cPickle.Pickler(2)
+        pickler = cPickle.Pickler()
         pickler.fast = 1
         return pickler.dump((msgid, flags, name, args), 1)