Remove trailing slashes from topic strings (review comment from Nicholas Humfrey)

Public review comment received from Nicholas Humfrey (point 8)

8) I do wish that leading and trailing slashes were forbidden. I think
topic names should either have structure and semantic meaning, or not.
The half-way house of 'use the topic name however you want' and slashes
being significant for pattern-matching is confusing.

Roger Light has responded this comment as follows:

Not that I'm disagreeing with you, but the leading/trailing slashes
just create zero length topic hierarchy element at the beginning/end
of the topic. So the topic "/mqtt//comment/" would be split into five
levels, '', 'mqtt', '', 'comment' and ''. The only inconsistency that
I am aware of is that although leading slashes allow you to produce a
topic that starts with a zero length element, there is no way to
address that zero length element itself because zero length topics are
forbidden for some reason.

Response from Nicholas:

Hm, interesting. Although it is logical, I doubt many users would
immediately understand that.