The fixes for #643424 was part of the fix for #652727, that's why both
fixes are pushed together.

- The cause for #643424 was the improper use of get_partial_join_cost(),
  which assumed that the 'n_tables' parameter was the upper bound for
  query plan node indexes.
  Fixed by generalizing get_partial_join_cost() as a method that computes
  the cost of any partial join.

- The cause of #652727 was that JOIN::choose_subquery_plan() incorrectly
  deleted the contents of the old keyuse array in the cases when an injected
  plan would not provide more key accesses, and reoptimization was not actually
  performed.