Deprecate OperationContext.getRootResource(); provide variant to get a read only view of an addressed branch of the Resource tree

OperationContext.getRootResource() is very expensive as it clones the entire model in order to provide a defensive copy that callers can't use to mutate parent resources.

In almost all cases the caller then navigates to a lower level of the tree to read. The OperationContext should provide a method to do this for them, restricting the cost of defending against writes to the portion of the tree the caller actually cares about.