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_base.h

@@ -267,7 +267,6 @@ bool open_normal_and_derived_tables(THD *thd, TABLE_LIST *tables, uint flags,
                                     uint dt_phases);
 bool lock_tables(THD *thd, TABLE_LIST *tables, uint counter, uint flags);
 int decide_logging_format(THD *thd, TABLE_LIST *tables);
-void free_io_cache(TABLE *entry);
 void intern_close_table(TABLE *entry);
 void kill_delayed_threads_for_table(TDC_element *element);
 void close_thread_table(THD *thd, TABLE **table_ptr);