short patch for bug #2184 'Prepared statements in embedded library was

broken with recent changes' (attempt 2).
Adding Statement_core is better because:
- set_statement() code is shorter and you don't need to modify it when adding
new members to Statement_core
- a bit faster (you don't have virtual call and don't free_root() twice)
Do that short patch instead in hope that set_statement() will be sooner or
later removed entirely

sql/sql_class.h

@@ -582,8 +582,7 @@ public:
   Statement_map stmt_map; 
   /*
     keeps THD state while it is used for active statement
-    Note, that double free_root() is safe, so we don't need to do any
-    special cleanup for it in THD destructor.
+    Note: we perform special cleanup for it in THD destructor.
   */
   Statement stmt_backup;
   /*