Keep service discovery information after the websocket disconnect for a while

[huan]

If a Puppet Service Server is restarting, the WebSocket connection to the io will be lost for a while.

We should keep the old information available for a period of time, so that:

1. It will be able to be discovered between the restarting of the service server,
2. It will timeout after a period of time.

The exact timeout number needs to be investigated. (we can refer to the configuration of etcd or zookeeper)

CC @windmemory