Mandate the use of separate id spaces for device normal topic alias's

Section changed and should read; 7.3 Topic Name Aliasing and Mapping On the gateway the mapping table between registered topic ids and topic names is implemented per client (and not by a single shared pool between all clients), to reduce the risk of an incorrect topic id from a client matching another client’s valid topic. For performance and efficiency reasons the broker may choose to align TopicIds for registered topic aliases between multiple clients. The mapping table of predefined topic aliases is separate from registered aliases. It is global and shared between all clients and gateways and may overlap with registered aliases, since it is considered to be in a different pool.