Exporter always returns the old value of the removed channel

[jurkust]

Exporter always returns the old value of the removed channel, just restart of exporter fixes this.

To reproduce:
Start NSQ and create a topic with few channels and add a few messages to that topic.
Start exporter and check what stats it returns.
Remove one of the channels and then check again what stats exporter returns.
You will see, that it still returns that remove channel with old depth.