ZeroMQTransport minor fixes
ZeroMQTransport minor fixes

- purge implemented (with timeout for possible race)
- fixed requeue
- in stop consume, only put consumer closed message if queue still exists (it may have been deleted and that's ok)
- get_stats now respects no consumers on a queue instead of asserting

crchemist/scioncc,ooici/pyon,mkl-/scioncc,ooici/pyon,crchemist/scioncc,mkl-/scioncc,scionrep/scioncc,mkl-/scioncc,scionrep/scioncc,crchemist/scioncc,scionrep/scioncc