Test fails sporadically and very rarely on this:
```
let $org_queries= `SHOW STATUS LIKE 'Queries'`;
SELECT f1();
CALL p1();
let $new_queries= `SHOW STATUS LIKE 'Queries'`;
let $diff= `SELECT SUBSTRING('$new_queries',9)-SUBSTRING('$org_queries',9)`;
```
if COM_QUIT from one of the earlier (in the test) disconnect's
happens between the two SHOW STATUS commands.
Because COM_QUIT increments "Queries".
The directly previous test uses wait_condition to wait for
its disconnects to complete. But there are more disconnects earlier
in the test file and nothing waits for them.
Let's change wait_condition to wait for *all* disconnect to complete.
Add variable max_used_connections_time to show the time at which
max_used_connections reached its current value. This is useful for
troubleshooting high connection counts. MySQL 8 has this already.
All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the BSD-new
license. I am contributing on behalf of my employer Amazon Web Services.
