Modify Language around use of $ topics

Section 4.7.2 Topics beginning with $ states 'The Server SHOULD prevent Clients from using such (where 'such' means topics beginning with $) Topic Names to exchange messages with other Clients'. We think this is a bit restrictive and should not necessarily be strictly enforced by the broker.

Instead we would prefer the 'SHOULD' be changed to a 'MAY'. This is consistent with the Non normative comment which states that "Applications cannot use a topic with a leading $ character for their own purposes".

We would prefer a language that leads broker implementations to may allow the use of $ topic namespaces through configurable options rather block such topics in an hard-coded way.