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

mysql-test/main/flush.test

@@ -566,17 +566,7 @@ create table t2 (i int);
 handler t1 open;
 
 connection con1;
---echo # Sending:
---send flush tables with read lock
-
-connection con2;
---echo # Wait until FTWRL starts waiting for 't1' to be closed.
-let $wait_condition=
-  select count(*) = 1 from information_schema.processlist
-  where state = "Waiting for table flush"
-  and info = "flush tables with read lock";
---source include/wait_condition.inc
-
+flush tables with read lock;
 connection default;
 --echo # The below statement should not cause deadlock.
 --echo # Sending:
@@ -591,9 +581,6 @@ let $wait_condition=
 --source include/wait_condition.inc
 
 connection con1;
---echo # FTWRL should be able to continue now.
---echo # Reap FTWRL.
---reap
 unlock tables;
 
 connection default;