BugTrack/2407 Make link files (cache/*.rel,*.ref) automatically

BugTrack/2407 Make link files (cache/*.rel,*.ref) automatically

* Remove related files (*.rel) and reference files (*.ref) in
  cache directory of the repository and distribute package
* Prepare page related links and references for links_get_related()

Make link files automatically only if the page count < 50.
Because large number of update links will cause PHP timeout.