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/lock.cc

@@ -77,6 +77,7 @@
 #include "sql_base.h"                       // close_tables_for_reopen
 #include "sql_parse.h"                     // is_log_table_write_query
 #include "sql_acl.h"                       // SUPER_ACL
+#include "sql_handler.h"
 #include <hash.h>
 #include "wsrep_mysqld.h"
 
@@ -1015,6 +1016,8 @@ bool Global_read_lock::lock_global_read_lock(THD *thd)
   {
     MDL_request mdl_request;
 
+    mysql_ha_cleanup_no_free(thd);
+
     DBUG_ASSERT(! thd->mdl_context.is_lock_owner(MDL_key::GLOBAL, "", "",
                                                  MDL_SHARED));
     mdl_request.init(MDL_key::GLOBAL, "", "", MDL_SHARED, MDL_EXPLICIT);