Adds the GET overall_buckets API.
The REST end point is: GET
/_xpack/ml/anomaly_detectors/job_id/results/overall_buckets
The API returns overall bucket results. An overall bucket
is a summarized bucket result over multiple jobs.
It has the `bucket_span` of the longest job's `bucket_span`.
It also has an `overall_score` that is the `top_n` average of the
max anomaly scores per job.
relates elastic/x-pack-elasticsearch#2693
Original commit: elastic/x-pack-elasticsearch@ba6061482d
