BibRank: fix update of rnkMETHOD.last_updated

• For some ranking methods such as citations/downloads, the rnkMETHOD's last_updated timestamp was updated either continuously during execution (in case of citation), or at the end of the process (case of tag-based ranking methods). This was generating various problems.
  
  This patch updates the timestamp correctly: that is, only at the end of the successful run of the ranking process, and with the value of the starting task time, so that if the process was interrupted by an upload in the middle, there would be no records skipped.
  
  (The word similarity ranking technique was not affected by this problem.)

• The citation data cacher timestamp verifier is now checking the rnkMETHOD last_updated timestamp, in order not to reload citation dictionaries while they are being rebuilt.