zmq4: socket close and {w,r}pool close

[sbinet]

as noted in #76 (comment) there's a bit of a confusing ownership between who's responsible for closing net.Conn between the underlying zmq4.socket and the various {r,w}pool implementations.

this leads to either non-executed code or "double-close" of connections.

we should probably make sure the {r,w}pool implementations have the complete ownership/responsibility for closing connections.