Add http/2 support

[GentlemanHal]

https://www.eclipse.org/jetty/documentation/current/http2-configuring-haproxy.html

https://github.com/sunng87/ring-jetty9-adapter

[joejag]

HAProxy 1.8 (nov 2017) now support this: https://www.haproxy.com/blog/whats-new-haproxy-1-8/

[GentlemanHal]

I think we also need to update the Nevergreen server otherwise it probably won't make much difference.

[joejag]

Hmm. Tried this tonight. Http2 only works on TLS connections. We are terminating the connection at the AWS load balancer before, so I can't enable the feature on haproxy.

Still though, we can do it at the Jetty level, so folk other than the official site can use it.

[GentlemanHal]

Apparently we do support http/2 now, I guess this is something AWS is giving us for free? I guess its also automatically downgrading to forward the requests on to the Nevergreen server?

I'm wondering if there is some way of enabling this so it is supported end to end, perhaps an AWS certificate we can import into the JVM so we don't have to terminate TLS?