Ask statsd plugin go count number of timed events

Other hidden options are:

* TimerUpper
* TimerLower
* TimerCount
* TimerSum

(From <https://github.com/collectd/collectd/blob/collectd-5.4.0/src/statsd.c#L613>)
Add other statsd timers, and associated carbon aggregation policies
Calculate 50th and 95th percentile for statds timers, not 90th.

We only had 90th here because the config example shows it. Thes values
are more useful
Aggregate only collectd's statsd timers in this manner
Merge pull request #18 from ministryofjustice/timer-counts

Ask collectd's statsd plugin to dervie events from timers