WIP 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>
`last_run_str` should be computed at the beginning of the bibcheck task execution, otherwise, if bibindex/webcoll are run during bibcheck we risk another race condition.

To the computation you should add as an upper limit the `task_get_task_param('task_starting_time')`.

Additionally, in case no collection/index is specified as a filter for the given rule, the previous behavior is still the best (in this way we indeed avoid recomputing twice bibcheck over the same records).