BibFormat: Read record_stats from ES (if enabled)

* Adds CFG_ELASTICSEARCH_SEARCH_HOST variable to invenio.conf that
  points to a host that can used to query with elasticsearch queries.
* Record stats are being loaded from elasticsearch if elasticsearch is
  enabled (if not, they are being loaded from DB as before).

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