Simple metrics

This removes all the `com.gu.management` types and friends (Or most of them);
 - `AbstractType[T]` - Buh bye.
 - `Metric`
 - `TimingMetric`
 - `CountMetric`

Some are still left behind for the time being;
 - `GaugeMetric`
 - `FrontendTimingMetric` in the current form

There are more simple new types;
 - `CountMetric`
 - `DurationMetric`

Some functionality has been removed. For example `measure(block: => T)`. This will be easy to reintroduce if needed, but nothing is actually using it; Any code that is actually timing things is doing it itself using `StopWatch` or via other methods.

This is the initial pull request in order to avoid having a massive PR.

#### Next Steps
 - ~~Remove most `GaugeMetric`~~
 - ~~Clean `FrontendTimingMetric`~~
 - Carry the metric types as far as the `CloudWatch` trait so we can use `StandardUnit` and `name`.
 - Use `StatisticSet` to know how many datapoints were used.

@gklopper