Log a debug message when two or more indices have same or very close cost amounts

The index selection mechanism in QueryImpl may lead to the wrong index to be picked up in case different indexes return same (or very close) costs.
As a remedy we can set cost overrides, change queries or index definitions.

However to speed up identification of such problematics situations it'd be good to have a line in the (debug) logs reporting when for a particular query a number of indexes have same or very close costs (in the order of 10^-1). This would give a first pointer to such possible problematic situations to be inspected.