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/handler.cc

@@ -30,7 +30,7 @@
 #include "sql_table.h"                   // build_table_filename
 #include "sql_parse.h"                          // check_stack_overrun
 #include "sql_acl.h"            // SUPER_ACL
-#include "sql_base.h"           // free_io_cache
+#include "sql_base.h"           // TDC_element
 #include "discover.h"           // extension_based_table_discovery, etc
 #include "log_event.h"          // *_rows_log_event
 #include "create_options.h"
@@ -5863,8 +5863,6 @@ int handler::ha_reset()
   DBUG_ASSERT(table->key_read == 0);
   /* ensure that ha_index_end / ha_rnd_end has been called */
   DBUG_ASSERT(inited == NONE);
-  /* Free cache used by filesort */
-  free_io_cache(table);
   /* reset the bitmaps to point to defaults */
   table->default_column_bitmaps();
   pushed_cond= NULL;