Overseer queue length metrics report the same value for all queues

We observe that the reported queue length metrics for the Overseer's state update and collection work queues are always identical.

I believe the cause is a shared instance of the Stats class that the Overseer injects into the ZkDistributedQueue instances for each of it's queues. The Stats class is only able to keep track of the length of one queue however, and this value would be clobbered whenever any of the ZkDistributedQueue instances updates it.