Evaluation for Cross Engine Join only multi-table queries processing

This issue describes a new feature enables processing of foreign engines queries, e.g. InnoDB. Consider a query

SELECT count(*) from non_mcs1 t1, non_mcs2 t2 where t1.c1 = t2.c1;

Given there is a mechanism to hand such queries over to MCS SELECT handler one has to alter a number of facilities to enable processing for multiple tables queries JOIN-ed.
This issue is supposed to deliver evaluation for JOINed foreign tables query performance, .e.g count() with an explicit GROUP BY uses key columns that don't belong to Innodb index.
The mentioned handover mechanism in the context of this issue would be a temporary shortcut that conditionally hands over processing of a query down to MCS.
As of now CrossEngineStep is an equivalent of TBPS that implements TupleDeliveryStep interface thus can be the source step of a JobList.