Update STALE and ANCIENT header attributes atomically

Update STALE and ANCIENT header attributes atomically

The ThreadSanitizer found a data race when updating the stale header.
Instead of trying to acquire the write lock and failing occasionally
which would skew the statistics, the dns_rdatasetheader_t.attributes
field has been promoted to use stdatomics.  Updating the attributes in
the mark_header_ancient() and mark_header_stale() now uses the cmpxchg
to update the attributes forfeiting the need to hold the write lock on
the tree.  Please note that mark_header_ancient() still needs to hold
the lock because .dirty is being updated in the same go.