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/binlog/t/binlog_dmls_on_tmp_tables_readonly.test

@@ -22,7 +22,6 @@
 DROP TABLE IF EXISTS t1 ;
 --enable_warnings
 
---enable_connect_log
 --echo # READ_ONLY does nothing to SUPER users
 --echo # so we use a non-SUPER one:
 CREATE USER test@localhost;
@@ -89,4 +88,3 @@ SET GLOBAL READ_ONLY=0;
 
 disconnect con1;
 DROP USER test@localhost;
---disable_connect_log

mysql-test/suite/binlog/t/binlog_empty_xa_prepared.test

@@ -3,6 +3,7 @@
 
 --source include/have_binlog_format_mixed.inc
 --source include/have_innodb.inc
+--source include/count_sessions.inc
 
 RESET MASTER; # clear binlogs
 # MDEV-22420 DDL on temporary object is prohibited when XA is in prepare state
@@ -126,7 +127,6 @@ DROP TABLE t1;
 CREATE TABLE t1 (a INT PRIMARY KEY) ENGINE=InnoDB;
 INSERT INTO t1 VALUES (1);
 
---source include/count_sessions.inc
 --connect(con1,localhost,root,,)
 
 XA START '1';
@@ -148,8 +148,6 @@ XA COMMIT '1';
 --source include/show_binlog_events.inc
 DROP TABLE t1;
 
---source include/count_sessions.inc
-
 --connect(con2,localhost,root,,)
 CREATE TABLE tm (a INT PRIMARY KEY) ENGINE=MyISAM;
 XA START '1';

mysql-test/suite/binlog/t/binlog_truncate_kill.test

@@ -46,7 +46,6 @@ connection con1;
 --source include/show_binlog_events.inc
 
 disconnect con1;
---source include/wait_until_disconnected.inc
 connection default;
 
 SELECT * FROM t1;

mysql-test/suite/binlog/t/innodb_rc_insert_before_delete.test

@@ -2,7 +2,6 @@
 --source include/have_debug.inc
 --source include/have_debug_sync.inc
 --source include/have_binlog_format_mixed.inc
---source include/count_sessions.inc
 
 RESET MASTER;
 
@@ -89,4 +88,3 @@ SELECT * FROM t;
 --disconnect pause_purge
 SET DEBUG_SYNC="RESET";
 DROP TABLE t;
---source include/wait_until_count_sessions.inc