Phase 3: Implementation of re-optimization of subqueries with injected predicates
           and cost comparison between Materialization and IN->EXISTS strategies.

The commit contains the following known problems:
- The implementation of EXPLAIN has not been re-engineered to reflect the
  changes in subquery optimization. EXPLAIN for subqueries is called during
  the execute phase, which results in different code paths during JOIN::optimize
  and thus in differing EXPLAIN messages for constant/system tables.
- There are some valgrind warnings that need investigation
- Several EXPLAINs with minor differences need to be reconsidered after fixing
  the EXPLAIN problem above.

This patch also adds one extra optimizer_switch: 'in_to_exists' for complete
manual control of the subquery execution strategies.