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/opt_range.h

@@ -26,6 +26,8 @@
 
 #include "records.h"                            /* READ_RECORD */
 #include "queues.h"                             /* QUEUE */
+#include "filesort.h"                           /* SORT_INFO */
+
 /*
   It is necessary to include set_var.h instead of item.h because there
   are dependencies on include order for set_var.h and item.h. This
@@ -1658,6 +1660,7 @@ QUICK_RANGE_SELECT *get_quick_select_for_ref(THD *thd, TABLE *table,
                                              ha_rows records);
 SQL_SELECT *make_select(TABLE *head, table_map const_tables,
 			table_map read_tables, COND *conds,
+                        SORT_INFO* filesort,
                         bool allow_null_cond,  int *error);
 
 bool calculate_cond_selectivity_for_table(THD *thd, TABLE *table, Item **cond);