Enable fast-import mode for headers from rpmdb

Enable fast-import mode for headers from rpmdb

- Assume our home turf is safe enough for this - in order to reach
  the rpmdb, headers must've gone through the more rigorous checking
  that's done through the rpmReadPackageFile() paths, plus in
  default configuration we'll be doing further verification on the
  header before loading the headers so the risk seems acceptable
  for the speed gain.