LP:615752 - Subquery caching is not visible in EXPLAIN

As discussed on the dev meeting, it is imperative that all optimizations are visible one way or another. Subquery caching is currently only detectable via SHOW STATUS LIKE 'Subquery_cache_hit' which does not scale well for automatic testing.

Instead "Using subquery cache" must be present in the Notes section of EXPLAIN , or alternatively some indication must be given in EXPLAIN EXTENDED.