Exception safety

[jonesmz]

I think that mqtt_cpp should document (with the noexcept keyword) the parts of the mqtt_cpp code that are exception safe.

For example, most of the member functions in property.hpp, and message.hpp and v5_message.hpp can be marked as noexcept.

while it would be a nice goal to make a decision about noexcept on every single function in mqtt_cpp, that's a lot of work.

I think targeting the easy functions is best. Simple ones that don't call other functions.