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_plugin.cc

@@ -1796,7 +1796,8 @@ static void plugin_load(MEM_ROOT *tmp_root)
     goto end;
   }
 
-  if (init_read_record(&read_record_info, new_thd, table, NULL, 1, 0, FALSE))
+  if (init_read_record(&read_record_info, new_thd, table, NULL, NULL, 1, 0,
+                       FALSE))
   {
     sql_print_error("Could not initialize init_read_record; Plugins not "
                     "loaded");