Before MDEV-24671, the wait time was derived from my_interval_timer() /
1000 (nanoseconds converted to microseconds, and not microseconds to
milliseconds like I must have assumed). The lock_sys.wait_time and
lock_sys.wait_time_max are already in milliseconds; we should not divide
them by 1000.

In MDEV-24738 the millisecond counts lock_sys.wait_time and
lock_sys.wait_time_max were changed to a 32-bit type. That would
overflow in 49.7 days. Keep using a 64-bit type for those millisecond
counters.

Reviewed by: Marko Mäkelä