zmq4: provide zmq4.Proxy

[sbinet]

we should have a way to expose something akin to zmq_proxy:

int zmq_proxy (const void *frontend, const void *backend, const void *capture);

The zmq_proxy() function starts the built-in ØMQ proxy in the current application thread.

The proxy connects a frontend socket to a backend socket. Conceptually, data flows from
frontend to backend. Depending on the socket types, replies may flow in the opposite
direction.
The direction is conceptual only; the proxy is fully symmetric and there is no technical
difference between frontend and backend.

Before calling zmq_proxy() you must set any socket options, and connect or bind both
frontend and backend sockets. The two conventional proxy models are:

zmq_proxy() runs in the current thread and returns only if/when the current context is
closed.

If the capture socket is not NULL, the proxy shall send all messages, received on both
frontend and backend, to the capture socket. The capture socket should be
 a ZMQ_PUB, ZMQ_DEALER, ZMQ_PUSH, or ZMQ_PAIR socket.