BibUpload: ingestion_date and creation_date

* Introduces new ingestion_date column to bibrec table, allowing
  independently keeping track of the time when a record is first
  ingested into the repository, from the bibliographic creation date
  that could be used to properly sort the record according to its
  corresponding document date.

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
WebSearch: new dateingested logical field

* Introduces new dateingested logical field in the search engine
  syntax, corresponding to bibrec.ingestion_date.

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
BibCheck: new creation_date check

* Exploits the BibCheck infrastructure to backdate the creation date of
  a record based on the actual content of its metadata. In this way
  tools such as BibSort will be able to properly sort records according
  to their real creation.
  (closes #1719) (closes #1604)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
BibSort: creation date and ingestion order

* Introduces proper sorting by creation date, exploiting
  new bibcheck plugin introduced in previous commit and
  consequently backward compatible "ingestion order"
  sorting.

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