sstate-mirror-sites: simplify, use downloadfilename

- Drop incorrect nativelsbstring dupe check (it's comparing an expanded string to an unexpanded value)
- Use downloadfilename to avoid cluttering up the root of SSTATE_DIR.
- Use eventmask to avoid running the event handler when we don't have to

Signed-off-by: Christopher Larson <chris_larson@mentor.com>