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/suite/handler/aria.result

@@ -1485,7 +1485,7 @@ handler t2 open;
 flush tables with read lock;
 handler t1 read next;
 a	b
-1	1
+2	1
 select a from t3;
 a
 1

mysql-test/suite/handler/heap.result

@@ -1484,7 +1484,7 @@ handler t2 open;
 flush tables with read lock;
 handler t1 read next;
 a	b
-1	1
+2	1
 select a from t3;
 a
 1

mysql-test/suite/handler/innodb.result

@@ -1489,7 +1489,7 @@ handler t2 open;
 flush tables with read lock;
 handler t1 read next;
 a	b
-1	1
+2	1
 select a from t3;
 a
 1

mysql-test/suite/handler/myisam.result

@@ -1485,7 +1485,7 @@ handler t2 open;
 flush tables with read lock;
 handler t1 read next;
 a	b
-1	1
+2	1
 select a from t3;
 a
 1