Tail: ingegrate percentile calculation from statsd module

  - move utils_latency.o from statsd.so into the collectd core, since now several modules need it
  - add new configuration value for DSType - GaugeLatency
  - add new configuration value Percentile which has similar meaning to the statsd 'TimerPercentile' setting
  - add new value-Type latency_gauges which transports Min/Max/Avg/Percentile value pairs to our writers