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
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