queues when the thread is not holding a space->latch.

When UNIV_DEBUG is defined while UNIV_SYNC_DEBUG is not,
latching order violations will still occur and deadlocks will be possible.

sync_thread_levels_nonempty_gen(): Renamed from
sync_thread_levels_empty_gen().  Return the violating latch or NULL
instead of FALSE or TRUE, except that there will be a ut_error before
the non-NULL return.

sync_thread_levels_empty_gen(): A macro that negates the return value of
sync_thread_levels_nonempty_gen().

sync_thread_levels_contains(): New function, based on
sync_thread_levels_nonempty_gen().

This should fix Issue #441.