Improve getManagementContext

- Create ManagementContextInternal. Adds the interface ManagementContextInternal. Remove references/casts to AbstractManagementContext; instead use ManagementContextInternal.

- Remove EntityManagementSupport.getManagementContext(boolean). Just have getManagementContext(), which will always return either the real or the "non-deployment" management context. Relies on failing fast by subsequent calls failing, if not permitted on the "non-deployment" management context.

- Don't deprecate AbstractEntity.getManagementContext()