setup: Use the number of completed password changes for race detection.
setup: Use the number of completed password changes for race detection.

The start time of the last password change was the wrong time to use,
because we could start a password change, start another request,
finish the password change, and then observe that the other request
failed due to the password change.

We could use the end time, but a counter is more robust to
sub-millisecond race conditions.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

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