UserContext should store locationId instead of the Location object

Storing hibernate objects in the http session is discouraged because of issues like lazy loading exceptions that may come up. For this particular case, Lazily loading Location.childLocations led into problems as per the discussion at: https://groups.google.com/a/openmrs.org/forum/#!topic/dev/OjzXttNgckM

So this ticket is about storing the locationId integer instead of the Location object in the UserContext, which is stored in the http session by OpenmrsFilter.