Fix ZMQ.Poll for NetMQ.Proxy

NetMQ.Proxy can't be started at all because ZMQ.Poll throws a null ref when called with an infinite timeout.
The null ref is on the stopwatch which is used to compute the current timeout but it was never instanciated in this case.
Moreover, the value returned is 0 when the timeout is negative so there is no way to have an actual infinite timeout on Select().
I also added a unit test on ZMQ.Poll(-1) and another one on a simple use case of NetMQ.Proxy.