update netty to 3.6.6.Final

netty 3.6.4 and 3.6.5 were bad releases, but 3.6.6 has been stable for a while now.

See
https://github.com/netty/netty/issues?milestone=44&page=1&state=closed
https://github.com/netty/netty/issues?milestone=49&page=1&state=closed
https://github.com/netty/netty/issues?milestone=50&page=1&state=closed
for details of the issues that have been closed since the 3.6.3 release.

This update to 3.6.6 passes ./runtests and other usage.