JOIN_CACHE has a light-weight initialization mode that's targeted at
EXPLAINs. In that mode, JOIN_CACHE objects are not able to execute.

Light-weight mode was used whenever the statement was an EXPLAIN. However
the EXPLAIN can execute subqueries, provided they enumerate less than
@@expensive_subquery_limit rows.

Make sure we use light-weight initialization mode only when the select is
more expensive @@expensive_subquery_limit.

Also add an assert into JOIN_CACHE::put_record() which prevents its use
if it was initialized for EXPLAIN only.