MDEV-21452: Replace all direct use of os_event_t

Let us replace os_event_t with mysql_cond_t, and replace the
necessary ib_mutex_t with mysql_mutex_t so that they can be
used with condition variables.

Also, let us replace polling (os_thread_sleep() or timed waits)
with plain mysql_cond_wait() wherever possible.

Furthermore, we will use the lightweight srw_mutex for trx_t::mutex,
to hopefully reduce contention on lock_sys.mutex.

FIXME: Add test coverage of
mariabackup --backup --kill-long-queries-timeout

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