BibCheck: collection-based filters and timestamps

* Store the earliest date among last time webcoll was run and the last
  bibindex `last_update` as the `last_run`, instead of the last time the
  rule was initiated. This prevents a race condition where records
  inserted between a webcoll run and the beginning of the following
  bibcheck ran were never considered by bibcheck.
  (resolves #2387)

Reported-by: Samuele Kaplun <samuele.kaplun@cern.ch>