Optimize flush tables with read lock (FTWRL) to not wait for select's

Part of MDEV-5336 Implement LOCK FOR BACKUP

The idea is that instead of waiting in close_cached_tables() for all
tables to be closed, we instead call flush_tables() that does:
- Flush not used objects in table cache to free memory
- Collect all tables that are open
- Call HA_EXTRA_FLUSH on the objects, to get them into "closed state"
- Added HA_EXTRA_FLUSH support to archive and CSV
- Added multi-user protection to HA_EXTRA_FLUSH in MyISAM and Aria

The benefit compared to old code is:
- FTWRL doesn't have to wait for long running read operations or
  open HANDLER's

sql/sql_base.h

@@ -290,6 +290,8 @@ void close_log_table(THD *thd, Open_tables_backup *backup);
 
 bool close_cached_tables(THD *thd, TABLE_LIST *tables,
                          bool wait_for_refresh, ulong timeout);
+void purge_tables(bool purge_flag);
+bool flush_tables(THD *thd);
 bool close_cached_connection_tables(THD *thd, LEX_CSTRING *connect_string);
 void close_all_tables_for_name(THD *thd, TABLE_SHARE *share,
                                ha_extra_function extra,