New benchmark target for Netty.

This required some reworking to the harness since Netty doesn't
want to be used with an ExecutorService.

NettyHttpClient [HTTP_11]
bodyByteCount=1048576 headerCount=20 concurrencyLevel=10
Requests per second: 597.1

OkHttp [HTTP_11]
bodyByteCount=1048576 headerCount=20 concurrencyLevel=10
Requests per second: 705.2
Merge pull request #501 from square/jwilson_0201_netty

New benchmark target for Netty.