Make queue length and HWM state observable

[sigiesec]

For a zmq_socket, the current queue length and HWM state should be observable by the application, such that this information can be integrated into application monitoring data. This might be implemented in either a push or pull way. A push way might extend the current socket monitor facility, a pull way might add new socket options that can be queried via zmq_getsockopt.

[sigiesec]

@jana-rapava I am not completely sure, but didn't you work on something related to this at last year's pre-FOSDEM Hackathon? Maybe we can talk/work on this next week in Brussels?

[bluca]

This was implemented by #3383