zeromq api v3

changes required for send and recv in version 3 of libzmq.

`socket-send!`/`recv!` no longer work if v3 of zeromq is installed, these are the changes required to make those functions work in my racket code. + some additional changes as per http://www.zeromq.org/docs:3-1-upgrade

tested with `rrclient.rkt`/`rrworker.rkt` & `wuclient.rkt`/`wuserver.rkt` examples from zguide.

learning racket so apologies if there are any silly errors.