remove legacyNamespace

The naming scheme will be changed again and I don't need backwards
compatibility.
Add metric type "low frequency timer".
Be clearer about what a gauge is.
Move processing_time metric together with other low freq timers.
move graphite backend stats to prefixStats + 'graphiteBackend'
Remove now unused statsd_metrics
Remove unused variable
Put metric type at the end of the metric name.

Also, remove globalPrefix and don't have the metric type postfixes be
configurable.
Delete all metrics after flush.

Use keepLastValue or "Draw Null as Zero" in graphite instead. To delete a
metric, stop sending it to statsd, wait one flush interval and delete the
whisper file.