mariadb-test: wait on disconnect

Remove one of the major sources of race condiitons in mariadb-test.
Normally, mariadb_close() sends COM_QUIT to the server and immediately
disconnects. In mariadb-test it means the test can switch to another
connection and sends queries to the server before the server even
started parsing the COM_QUIT packet and these queries can see the
connection as fully active, as it didn't reach dispatch_command yet.

This is a major source of instability in tests and many - but not all,
still less than a half - tests employ workarounds. The correct one
is a pair count_sessions.inc/wait_until_count_sessions.inc.
Also very popular was wait_until_disconnected.inc, which was completely
useless, because it verifies that the connection is closed, and after
disconnect it always is, it didn't verify whether the server processed
COM_QUIT. Sadly the placebo was as widely used as the real thing.

Let's fix this by making mariadb-test `disconnect` command _to wait_ for
the server to confirm. This makes almost all workarounds redundant.

In some cases count_sessions.inc/wait_until_count_sessions.inc is still
needed, though, as only `disconnect` command is changed:

 * after external tools, like `exec $MYSQL`
 * after failed `connect` command
 * replication, after `STOP SLAVE`
 * Federated/CONNECT/SPIDER/etc after `DROP TABLE`

and also in some XA tests, because an XA transaction is dissociated from
the THD very late, after the server has closed the client connection.

Collateral cleanups: fix comments, remove some redundant statements:
 * DROP IF EXISTS if nothing is known to exist
 * DROP table/view before DROP DATABASE
 * REVOKE privileges before DROP USER
 etc

mysql-test/suite/innodb/t/innodb-index-online.test

@@ -13,9 +13,6 @@ SELECT name, count FROM INFORMATION_SCHEMA.INNODB_METRICS WHERE subsystem = 'ddl
 
 call mtr.add_suppression("InnoDB: Warning: Small buffer pool size");
 
-# Save the initial number of concurrent sessions.
---source include/count_sessions.inc
-
 CREATE TABLE t1 (c1 INT PRIMARY KEY, c2 INT, c3 TEXT)
 ENGINE=InnoDB STATS_PERSISTENT=0;
 INSERT INTO t1 VALUES (1,1,''), (2,2,''), (3,3,''), (4,4,''), (5,5,'');
@@ -537,10 +534,6 @@ DROP TABLE t1;
 
 SET DEBUG_SYNC = 'RESET';
 
-# Check that all connections opened by test cases in this file are really
-# gone so execution of other tests won't be affected by their presence.
---source include/wait_until_count_sessions.inc
-
 --disable_warnings
 SET GLOBAL innodb_monitor_enable  = default;
 SET GLOBAL innodb_monitor_disable = default;