general: Elasticsearch logging with Lumberjack.

* Added disabled-by-default config options for Elasticsearch logging.

* When Elasticsearch logging is configured, don't populate rnkPAGEVIEWS and
  rnkDOWNLOADS.

Signed-off-by: Joe MacMahon <joe.macmahon@cern.ch>
WebStat: register custom events on es