BibCatalog: Changed ticket_id type to string

* Changes the ticket_id returned by the RT ticketing system from int
  to string, so it's homogeneous with email ticketing system.

* Fixes the error in string formatting in oai_harvest_daemon.

* Amends the documentation for several tickets' related functions.

Signed-off-by: Sebastian Witowski <sebastian.witowski@cern.ch>