Removed TABLE->sort to make it possible to have multiple active calls to

filesort and init_read_record() for the same table.
This will simplify code for WINDOW FUNCTIONS (MDEV-6115)

- Filesort_info renamed to SORT_INFO and moved to filesort.h
- filesort now returns SORT_INFO
- init_read_record() now takes a SORT_INFO parameter.
- unique declaration is moved to uniques.h
- subselect caching of buffers is now more explicit than before
- filesort_buffer is now reusable even if rec_length has changed.
- filsort_free_buffers() and free_io_cache() calls are removed
- Remove one malloc() when using get_addon_fields()

Other things:
- Added --debug-assert-on-not-freed-memory option to make it easier to
  debug some not-freed-memory issues.

sql/sql_select.h

@@ -32,6 +32,7 @@
 #include "sql_array.h"                        /* Array */
 #include "records.h"                          /* READ_RECORD */
 #include "opt_range.h"                /* SQL_SELECT, QUICK_SELECT_I */
+#include "filesort.h"
 
 /* Values in optimize */
 #define KEY_OPTIMIZE_EXISTS		1
@@ -236,6 +237,7 @@ typedef struct st_join_table {
     For join tabs that are inside an SJM bush: root of the bush
   */
   st_join_table *bush_root_tab;
+  SORT_INFO *filesort;
 
   /* TRUE <=> This join_tab is inside an SJM bush and is the last leaf tab here */
   bool          last_leaf_in_bush;