- don't call JOIN::join_free(1) twice for every join in JOIN::cleanup().

The reason it happened was that both, JOIN::cleanup() and JOIN::join_free(),
went over all nested joins and called cleanup/join_free for them.
For that:
- split recursive and non-recursive parts of JOIN::cleanup() and
JOIN::join_free()
- rename JOIN::cleanup to JOIN::destroy, as it actually destroys its
  argument
- move the recursive part of JOIN::cleanup to st_select_lex::cleanup
- move the non-recursive part of JOIN::join_free to the introduced
  method JOIN::cleanup().

sql/sql_select.h

@@ -325,7 +325,7 @@ class JOIN :public Sql_alloc
   int optimize();
   int reinit();
   void exec();
-  int cleanup();
+  int destroy();
   void restore_tmp();
   bool alloc_func_list();
   bool make_sum_func_list(List<Item> &all_fields, List<Item> &send_fields,
@@ -349,7 +349,15 @@ class JOIN :public Sql_alloc
   int rollup_send_data(uint idx);
   int rollup_write_data(uint idx, TABLE *table);
   bool test_in_subselect(Item **where);
+  /*
+    Release memory and, if possible, the open tables held by this execution
+    plan (and nested plans). It's used to release some tables before
+    the end of execution in order to increase concurrency and reduce
+    memory consumption.
+  */
   void join_free(bool full);
+  /* Cleanup this JOIN, possibly for reuse */
+  void cleanup(bool full);
   void clear();
   bool save_join_tab();
   bool send_row_on_empty_set()