Implementing Scheduler SLA metrics.

High level overview:
- MetricCalculator runs periodically (every minute), pulls all task history,
  packages it into SlaInstance list and updates stats;
- Stat calculation is handled by a pair of: SlaAlgorithm and a set of
  applicable SlaGroups (logical groupings by job, cluster, resource and etc.);
- Stat name is generated by combining group and algorithm name parts.

Bugs closed: AURORA-293

Reviewed at https://reviews.apache.org/r/20398/