BibIndex: new index 'itemcount'

* Creates a new index 'itemcount' and plugs it to bibindex.
* Amends search_engine to work with 'itemcount' index
  for plus and span queries.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>