Revert "Upgrade to async-http-client 1.9.3"

This reverts commit aa4f4bf4b166e3bcff44caff3ddf3d487bb2b9bc.
Downgrade to Netty 3.9.6.Final

The only difference between Netty 3.9.6.Final and 3.10.0.Final is the removal
of deprecated methods. Unfortunately async-http-client still uses some of those
methods and we don't want to upgrade right now.