plex: Don't overwrite primary database on restart

This change fixes two major issues:

  1. If you don't use SIGQUIT to stop Plex it will corrupt its own
     database :(

  2. Newer versions of Plex keep metadata in the
     `com.plexapp.plugins.library.db` database.  This is the file that
     we copy into `/var/lib/plex/.skeleton`.  If we copy the empty
     database on top of this one the user will lose their entire
     library metadata.  This change skips the copy if the file
     already exists.