Update ptiff sync to prepend new views and improve behavior

* Removed the resync option to sync, as the resync function already takes
care of deleting all existing images, views, and sessions
* Removed the documentation statement about sync deleting any images that
were missing, as this never actually occured and would be dangerous to
implement if a temporary file system problem made images appear unavailable
* If an image is missing, it will now always be added, even if its
modification time was before the last recorded sync time
* Deleting old views when an existing image is requires updating is made
more efficient, only being attempted when necessary
Merge branch 'ptiff_sync_prepend'

* ptiff_sync_prepend:
  Update ptiff sync to prepend new views and improve behavior