Update the zmq::messsage_t::str() method to allow to print larger messages

[cacharle]

There is an arbitrary limitation of 1000 size to print the [too big to print] placeholder:

https://github.com/zeromq/cppzmq/blob/3bcbd9dad2f57180aacd4b4aea292a74f0de7ef4/zmq.hpp#L707C9-L707C28

It would be nice to have this limit as an argument instead:

    std::string str(size_t max_size = 1000) const

[gummif]

Agreed. Another improvements would be to print everything up to max_size.