zmq3 split HWM into SNDHWM/RCVHWM. Closes #246

Fixes #246. In zeromq3, the hwm (high watermark) socket option has been split into sndhwm/rcvhwm. Tested on CentOS 6 with zeromq3-3.2.4-1.el6 and python-zmq-2.2.0.1-1.el6 packages.