Slightly tweak how the metrics are calculated, condense into a single loop

[Crim]

This PR slightly tweaks your metric collection loops in the following way:

• It condenses the two loops into a single loop,
• Renames some of the variables to be more explicit
• Re-uses the result of getCurrentState() instead of calling it once per loop iteration
• Adds "currentOffset" metric
• Renames the metric keyspace "topic.[topic].partition.[partition].[metricName]" so its easier to browse related metrics in something like grafana/graphite.
• Updates changelog <-- May want to review and make any other changes here that I missed.

[stanlemon]

@Crim should we drop the other currentOffset metric then?

[Crim]

I considered it, I think for now lets just leave it. I think we should talk at some point in which (if any) of the SpoutProgress metrics are useful. I know you hate that class and want it to die, and I'm starting to agree with you :p