WL#2889: Add ha_extra(read-next-not-same)

Affects: Server-7.1   —   Status: Assigned   —   Priority: Medium

The task is to add ha_extra(read-next-not-same). 
This will allow us to do DISTINCT much faster and 
allow us to skip rows when we are doing distinct and 
only accessing fields that are part of the current index.

Code cleanup task requested by Monty.