Worker stats are reset when workers reconnect through websocket

[zhengyao]

The problem is in the websocket handler

broadway/broadway/api/handlers/worker_ws.py

Line 38 in 09abe45

 def handler_register(self, hostname): 

It's creating a completely new model.WorkerNode every time which resets a bunch of fields to their initial values. The correct behavior should be first checking if there exists one node with the given id then only update altered fields (hostname, etc.)