Track Tornado handlers by uniquely assigned IDs rather than objects.

This is early preparation for splitting apart Tornado into a queue
server and a frontend server.

zerver/lib/handlers.py

@@ -0,0 +1,12 @@
+current_handler_id = 0
+handlers = {}
+
+def get_handler_by_id(handler_id):
+    return handlers[handler_id]
+
+def allocate_handler_id(handler):
+    global current_handler_id
+    handlers[current_handler_id] = handler
+    ret = current_handler_id
+    current_handler_id += 1
+    return ret