[#508] Compute lat/lon of clusters as an average over all points
[#509] use clusterId as keyId to uniquely identify clusters.

* use new detailsId field to convey the surveyedLocaleId to the client.
[#510] Make placemark keyId unique across clusters and locales
[#508] Use primitives for constants
[#363] Move adapt cluster data to task
[#363] Remove unused code
Merge pull request #511 from akvo/hotfix/1.6.14

Hotfix/1.6.14