Very simple patch adding zmq_disconnect support

ZeroMQ 3.2 adds support for a new function `zmq_disconnect`. This is very useful in case one of the connected peers of a load-balanced socket drops, to avoid having to rebuild the whole socket in order to keep the load-balancing sane.

This patch is very small and backward compatible. In case the function isn't available (ZeroMQ < 3.2), it does nothing.