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/perfschema/r/one_thread_per_con.result

@@ -40,10 +40,7 @@ drop table test.t1;
 drop table test.t2;
 drop table test.t3;
 update performance_schema.setup_instruments set enabled='YES';
-connection con1;
 disconnect con1;
-connection con2;
 disconnect con2;
-connection con3;
 disconnect con3;
 connection default;

mysql-test/suite/perfschema/r/read_only.result

@@ -30,7 +30,9 @@ GRANT USAGE ON *.* TO `pfsuser`@`localhost`
 GRANT SELECT, UPDATE ON `performance_schema`.* TO `pfsuser`@`localhost`
 select * from performance_schema.setup_instruments;
 update performance_schema.setup_instruments set enabled='NO';
+ERROR HY000: The MariaDB server is running with the --read-only=ON option so it cannot execute this statement
 update performance_schema.setup_instruments set enabled='YES';
+ERROR HY000: The MariaDB server is running with the --read-only=ON option so it cannot execute this statement
 connection default;
 grant READ_ONLY ADMIN on *.* to pfsuser@localhost;
 flush privileges;

mysql-test/suite/perfschema/r/socket_connect.result

@@ -154,10 +154,8 @@ Expect 2
 # 5.0 Drop the client connections
 #==============================================================================
 # 5.1 Disconnect con1
-connection con1;
 disconnect con1;
 # 5.2 Disconnect con2
-connection con2;
 disconnect con2;
 connection default;
 #==============================================================================

mysql-test/suite/perfschema/r/socket_instances_func.result

@@ -199,11 +199,8 @@ wait/io/socket/sql/client_connection	<LOCALHOST>
 wait/io/socket/sql/client_connection	<LOCALHOST>
 wait/io/socket/sql/client_connection	
 connection default;
-connection con1;
 disconnect con1;
-connection con2;
 disconnect con2;
-connection con3;
 disconnect con3;
 connection default;
 # After waiting a bit we should have no differences to socket_instances

mysql-test/suite/perfschema/r/socket_summary_by_event_name_func.result

@@ -117,12 +117,10 @@ consistent with totals in socket_summary_by_instance
 #
 # 5.1 Disconnect con1
 #
-connection con1;
 disconnect con1;
 #
 # 5.2 Disconnect con2
 #
-connection con2;
 disconnect con2;
 #
 # 5.3 Drop mysqltest

mysql-test/suite/perfschema/r/socket_summary_by_instance_func.result

@@ -219,4 +219,3 @@ connection default;
 #    It must reset all counters.
 TRUNCATE TABLE performance_schema.socket_summary_by_instance;
 # 6. Cleanup
-connection default;

mysql-test/suite/perfschema/r/threads_history.result

@@ -951,10 +951,6 @@ where THREAD_ID = $con4_thread_id;
 count(*)
 0
 connection default;
-revoke all privileges, grant option from user1@localhost;
-revoke all privileges, grant option from user2@localhost;
-revoke all privileges, grant option from user3@localhost;
-revoke all privileges, grant option from user4@localhost;
 drop user user1@localhost;
 drop user user2@localhost;
 drop user user3@localhost;

mysql-test/suite/perfschema/r/transaction_nested_events.result

@@ -1272,7 +1272,6 @@ DELETE FROM db.t1;
 #=======================================================================
 # Cleanup
 #=======================================================================
-connection con1;
 disconnect con1;
 connection default;
 DROP DATABASE db;