DDS15 — No specification for unregister_type for TypeSupport interface

Currently there is no specification on the TypeSupport interface for an unregister_type operation as a companion to the register_type operation. This leads to variation between vendor implementations with respect to the lifecycle of type registrations. In the case of component-based architectural approaches it is desirable to be able to unload components to free resources. With no clear ability to unregister a type, it is not safe to unload component libraries while a container or manager of DDS infrastructure may still hold a reference to a no longer used type.