cqle: add UDTValueManager to detect sub-field updatees

UDTs are still updated en masse when changed. This aligns
with concept of 'frozen' types.

cassandra/cqlengine/columns.py

@@ -859,6 +859,16 @@ class Map(BaseContainerColumn):
         return [self.key_col, self.value_col]
 
 
+class UDTValueManager(BaseValueManager):
+    @property
+    def changed(self):
+        return self.value != self.previous_value or self.value.has_changed_fields()
+
+    def reset_previous_value(self):
+        self.value.reset_changed_fields()
+        self.previous_value = copy(self.value)
+
+
 class UserDefinedType(Column):
     """
     User Defined Type column
@@ -870,6 +880,8 @@ class UserDefinedType(Column):
     Please see :ref:`user_types` for examples and discussion.
     """
 
+    value_manager = UDTValueManager
+
     def __init__(self, user_type, **kwargs):
         """
         :param type user_type: specifies the :class:`~.UserType` model of the column

cassandra/cqlengine/usertype.py

@@ -59,6 +59,13 @@ class BaseUserType(object):
     def __str__(self):
         return "{{{}}}".format(', '.join("'{}': {}".format(k, getattr(self, k)) for k, v in six.iteritems(self._values)))
 
+    def has_changed_fields(self):
+        return any(v.changed for v in self._values.values())
+
+    def reset_changed_fields(self):
+        for v in self._values.values():
+            v.reset_previous_value()
+
     @classmethod
     def register_for_keyspace(cls, keyspace):
         connection.register_udt(keyspace, cls.type_name(), cls)