add lazy_property class to gevent.util

gevent/util.py

@@ -42,3 +42,15 @@ class wrap_errors(object):
         return getattr(self.func, item)
 
 
+class lazy_property(object):
+
+    def __init__(self, calculate_function):
+        self._calculate = calculate_function
+
+    def __get__(self, obj, _=None):
+        if obj is None:
+            return self
+        value = self._calculate(obj)
+        setattr(obj, self._calculate.func_name, value)
+        return value
+