Publish on multiple topics

[edoardesd]

Since analyse how the broker reacts on several publish also on different topics could be interesting, I added the following feature:

• numTopics

With numTopics the number of different topics can be chosen launching mqtt-benchmark. The client will randomly publish on a topic. The code adds a random number to the current topic, creating a second level (e.g. "/topic/39").

[krylovsk]

After giving it another thought, I think implemented as this it can be confusing to the user because of the different combinations of # of clients and the # of topics.

Can you describe your use case in more detail? Let's continue this discussion in #5 (will keep PR open for now)