Fix Python3 bug with Timers and heaps

Add comparison operator to allow Timers with the same end time to be
pushed into heaps. It's poorly documented, but the warning can be found
here:
https://docs.python.org/2/library/heapq.html#priority-queue-implementation-notes
"In the future with Python 3, tuple comparison breaks for (priority,
task) pairs if the priorities are equal and the tasks do not have a
default comparison order."

cassandra/connection.py

@@ -1020,6 +1020,9 @@ class Timer(object):
         if timeout < 0:
             self.callback()
 
+    def __lt__(self, other):
+        return self.end < other.end
+
     def cancel(self):
         self.canceled = True