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_union.cc

@@ -173,7 +173,8 @@ select_union::create_result_table(THD *thd_arg, List<Item> *column_types,
 
 
 /**
-  Reset and empty the temporary table that stores the materialized query result.
+  Reset and empty the temporary table that stores the materialized query
+  result.
 
   @note The cleanup performed here is exactly the same as for the two temp
   tables of JOIN - exec_tmp_table_[1 | 2].
@@ -183,8 +184,6 @@ void select_union::cleanup()
 {
   table->file->extra(HA_EXTRA_RESET_STATE);
   table->file->ha_delete_all_rows();
-  free_io_cache(table);
-  filesort_free_buffers(table,0);
 }