The "executor part" is a new class QEP_TAB. JOIN_TABs are dropped at end
of JOIN::optimize. EXPLAIN and JOIN::exec use only QEP_TABs. So does the
construction of the AQP.
QEP_TAB and JOIN_TAB share certain members, put in a class QEP_shared.
SQL_SELECT is removed, most of it goes into QEP_shared or QEP_TAB.
The memcpy of all JOIN_TABs at end of get_best_combination() is gone;
instead, we keep the old JOIN_TABs for a bit longer, but access them
in the final plan's order thanks to best_ref[] pointers.
This refactoring fixes
Bug#18921626 DEBUG CRASH IN PLAN_CHANGE_WATCHDOG::~PLAN_CHANGE_WATCHDOG AT SQL_OPTIMIZER.CC
and Bug#18535226 DEBUG CRASH ON QUICK_RANGE_SELECT::RESET.