Logbook events endpoint to retrieve dates + search by date

related to E-DataPortal#527 (closed)

• add new end point GET /logbook/:sessionId/event/dates which returns the list of events creationDate for an investigationId or an instrumentName. It should also take care of the different types of events, and the search criteria. It returns an array of {_id : "YYYY-MM-DD", event_count: n}

• GET /logbook/:sessionId/event/ add date param to filter on creationDate event