Reuse more components from statsd exporter

[glightfoot]

The graphite and statsd exporters diverged long ago, but basically do the same thing with different line processing and collection (all metrics are gauges and only use the latest value). The exporter should be restructured to use as much as possible from the statsd exporter, with packages replacing the important parts.

[matthiasr]

Restructuring happened in #144. A next step could be to transition to a common collector. For that, the first thing would be to catalogue the behaviors, and see what needs to be changed or reconciled.