disable limit optimization)

Bug#31686878 - BACKPORT MAIN BUG 30348211 TO RELEASE 5.7

The optimization to switch from a non-ordering index to an ordering
index for group by and order by when there is a limit clause makes wrong
decisions for some queries for various reasons.

This patch introduces an optimizer switch that enables users to disable
this optimization if the switch to ordering index results in a bad plan.

The name of the switch is "prefer_ordering_index". By default the switch
is on which means if an ordering index is found to be better than an
already chosen plan then optimizer will switch to the ordering
index. However if "prefer_ordering_index" is set to off, the switch to
ordering index because of the "limit optimization" will not happen,
resulting in optimizer picking the original plan which could have been
using a filtering index.

Change-Id: I511c586ac820cea8a63156299c08ee2213295130