Remove unnecessary long/StatusCode conversions

In preparation for a larger re-design of StatusCode clean up a few
places where the code unnecessarily relies on an implicit long
type-conversion. In detail:

- ObjectVector::remove: Return the index of the deleted object as
  documented in the base class. Before it always returned 1
- RegistryEntry: Change return type of some methods from long to
  StautsCode
- IODataManager: Get rid of the S_OK/S_ERROR enum alias as this relies
  on the actual type of the StatusCode enums
- RootDataConnection: Change return type of setCompression from
  long to StatusCode