BibUpload: bibrec timestamp bug

• In 70f76183f018c1a3bb7313c39b6a4ce8e5e908a4 bibupload was modified to store as record modification time the initial timestamp of when the modification began, rather than the latest timestamp of when modification finished (as it was before this patch). This might lead to all sorts of malfunctions if tasks such as bibindex, bibrank, refextract, bibauthorid, etc. are run at the same time of a bibupload. This commit partially reverts this behaviour by indeed storing only the very latest timestamp into the bibrec table (so that other bibtasks are not missing the update). (fixes #1431)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>