Let us use a single rw_lock_t::wait_mutex and two condition
variables wait_cond, wait_cond_ex instead of two os_event_t
(which wrap a mutex and condition variable and some other data)

sync_array_wait_event(): Implement new waiting rules for rw_lock_t.
FIXME: Sometimes, rw_lock_s_lock() hangs here. TODO: Find out
what the lock->lock_word is in those cases. Maybe we should
treat not only the value 0 but also X_LOCK_HALF_DECR specially.