Add support for SSL passthrough

[aschepis]

It would simplify the configuration of some deployments of linkerd had the option of passing through SSL connections instead of trying to terminate and route it.

In my case I have a kubernetes cluster running l5d as a daemonset. The outgoing port is used to resolve both internal (k8s services) and external (AWS APIs, etc) endpoints. Not all tools/applications respect the conventional http_proxy and https_proxy environment variables. For instance JRuby (9.1.7.0) does not look at the https_proxy environment variable and just uses http_proxy. As a result, you can run into scenarios where SSL is sent to the http_proxy.

When this occurs, linkerd forwards along the CONNECT HTTP method (https://en.wikipedia.org/wiki/HTTP_tunnel#HTTP_CONNECT_tunneling) to the endpoint (e.g. AWS) resulting in a 400 Bad Request.

[adleong]

linkerd-tcp can already do this.

Adding this functionality to linkerd is more difficult and would require finagle and netty changes.