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/csv/flush.result

@@ -0,0 +1,25 @@
+CREATE TABLE t1(a INT NOT NULL) ENGINE=csv;
+INSERT INTO t1 VALUES(1);
+connect con1, localhost, root;
+LOCK TABLE t1 READ;
+connection default;
+FLUSH TABLES WITH READ LOCK;
+UNLOCK TABLES;
+# Must return 1 row
+SELECT * FROM t2;
+a
+1
+SELECT * FROM t1;
+a
+1
+connection con1;
+UNLOCK TABLES;
+connection default;
+INSERT INTO t2 VALUES(2);
+INSERT INTO t2 VALUES(2);
+SELECT * from t1,t2;
+a	a
+1	1
+1	2
+1	2
+DROP TABLE t1, t2;