#982 add dedupe.getstatistics api

Per gitlab the intent is to add a bunch of api so get the logic out of the form into somewhere more accessible.

Following agreement on this I would change all calls to CRM_Dedupe_Merger::getMergeStats to call this api
and get rid of the call to CRM_Core_BAO_PrevNextCache::retrieve from getMergeStats since that retrieve
function really does differrent stuff & is being kinda pointlessly overloaded here.