Fix race condition on check_stale_header

This commit fix a race that could happen when two or more threads have failed to refresh the same RRset, the threads could simultaneously attempt to update the header->last_refresh_fail_ts field in check_stale_header, a field used to implement stale-refresh-time.

By making this field atomic we avoid such race.

(cherry picked from commit c75575e3)

Closes #2442 (closed)