Many small changes

1. Fixed release date in notes for v4.0.6
2. Fixed bug in vs_netmq throughput test
3. Reverted unnecessary changes to Socket finalization process
4. Reverted inappropriate change to Socket.recv method
5. Added another zguide example (mtrelay)

Items #3 & #4 warrant further explanation, as they undo commits made by @hammett (see: comments on issues #68 and #69 for more details).