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.
Add MockWebServer.setNpnProtocols
Add instructions to execute the benchmark with correct JVM parameters.
Use the pooled allocator for performance reasons
Merge pull request #502 from adriancole/mws-npn

Tell MockWebServer which NPN protocols to use.
Use Caliper for benchmarking.
Flush control frames; don't flush user frames.
Merge pull request #504 from square/jwilson_0201_caliper

Use Caliper for benchmarking.
Merge pull request #505 from adriancole/flush-control-frames-dont-flush-user-frames

Flush control frames; don't flush user frames.
Merge pull request #503 from normanmaurer/pooled

Use the pooled allocator for performance reasons