1. Changed the lazy optimization for subqueries that can be
   materialized into bottom-up optimization during the optimization of
   the main query.

   The main change is implemented by the method
   Item_in_subselect::setup_engine.
  
   All other changes were required to correct problems resulting from
   changing the order of optimization. Most of these problems followed
   the same pattern - there are some shared structures between a
   subquery and its parent query. Depending on which one is optimized
   first (parent or child query), these shared strucutres may get
   different values, thus resulting in an inconsistent query plan.

2. Changed the code-generation for subquery materialization to be
   performed in runtime memory for each (re)execution, instead of in
   statement memory (once per prepared statement).
   - Item_in_subselect::setup_engine() no longer creates materialization
     related objects in statement memory.
   - Merged subselect_hash_sj_engine::init_permanent and
     subselect_hash_sj_engine::init_runtime into
     subselect_hash_sj_engine::init, which is called for each
     (re)execution.
   - Fixed deletion of the temp table accordingly.


mysql-test/r/subselect_mat.result:
  Adjusted changed EXPLAIN because of earlier optimization of subqueries.