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/table_cache.h

@@ -88,7 +88,6 @@ extern bool tdc_remove_table(THD *thd, enum_tdc_remove_table_type remove_type,
                              const char *db, const char *table_name,
                              bool kill_delayed_threads);
 
-
 extern int tdc_wait_for_old_version(THD *thd, const char *db,
                                     const char *table_name,
                                     ulong wait_timeout, uint deadlock_weight,
@@ -102,6 +101,7 @@ extern uint tc_records(void);
 extern void tc_purge(bool mark_flushed= false);
 extern void tc_add_table(THD *thd, TABLE *table);
 extern void tc_release_table(TABLE *table);
+extern TABLE *tc_acquire_table(THD *thd, TDC_element *element);
 
 /**
   Create a table cache key for non-temporary table.