Allow to stop Handler when waiting for a new request on ZMQ

[paneq]

Use poller and send ZMQ message to stop ? Send special kind of message to PULL socket used by underlying Connection ?

[paneq]

This is currently working when using signals for running application. However the additional usecase that I have in mind is that someone might be running a handler in separate thread. The handler is waiting for a request and we would like it to stop for some reasone. No signal was send.