Store broker offset history

[toddpalino]

For partitions that receive data very slowly, it is possible for Burrow to falsely alert for stopped partitions. This case is described in #303 (the second case). This is the starting state:

1. There has been no data in a long time
2. The currentLag is zero
3. The partition would be considered stopped by timestamps (if currentLag was not zero)
4. The consumer is running properly

Then the following things happen:

1. A new message comes in
2. The broker offset is updated in Burrow
3. A status check is performed in Burrow for the consumer - this reports stopped for this partition, as the lag is non-zero and the timestamps say it is stopped

To get around this, we need to wait a little bit to give the consumer a chance to see the new message and commit an updated offset. We can do this by storing the recent browser offsets that Burrow has gotten for the partition. If one of those recent offsets would have had the consumer at zero lag, we don't consider the partition to be stopped. If the interval config is 10, and the cluster offset-refresh is 30 seconds, this would delay the partition being marked as stopped by 5 minutes.

This does not expose the broker offset history to the user, or use it in any other way except when the partition might be considered stopped.