MEDIUM: Add support for "mode tcp"

[daniel-corbett]

New frontends and backends only had support for using models.FrontendModeHTTP
and models.BackendModeHTTP which limited them to "mode http" only. This commit
adds support for "mode tcp" using both upstream and downstream specific
configuration options. It defaults to http.

To use add a "config" option within an upstream similar to the below example:

Sample downstream service file for Redis

{
  "service": {
    "name": "redis",
    "port": 6379,
    "connect": {
      "sidecar_service": {
	"proxy" : {
	  "config": { "protocol" : "tcp" }
	}
      }
    }
  }
}

Sample upstream configuration:

  "upstreams": [{
 "destination_name": "redis",
 "local_bind_port": 6379,
 "config": { "protocol" : "tcp" }
  }

[pierresouchay]

Sounds Ok to me, we have to add to CHANGELOG.md (see #23) that this is a breaking change (since using TCP instead of HTTP by default now).
Since it is intended to mimic Envoy's Consul Connect, it seems legit to me.
@ShimmerGlass Any comment

[daniel-corbett]

Sounds Ok to me, we have to add to CHANGELOG.md (see #23) that this is a breaking change (since using TCP instead of HTTP by default now).
Since it is intended to mimic Envoy's Consul Connect, it seems legit to me.
@ShimmerGlass Any comment

Just to be clear this won't change the default to TCP yet. I was recommending that we make that change in the future after accepting this PR. For now, this just adds TCP but keeps default to HTTP

[pierresouchay]

@dcorbett-haproxy yes, you are right, that's only when you'll default it to TCP in a similar way as Envoy's. Let's wait for @ShimmerGlass to review it, but I think we are good.

[pierresouchay]

Hello @dcorbett-haproxy ,

Since we now run the unit tests on Travis (with end-to-end tests), can you please rebase with origin ?
git pull -r origin master && git push --force ?

Thank you

[ShimmerGlass]

There should be a type assertion here. If "protocol" is not a string it will crash.

[daniel-corbett]

Thanks for catching this. I added a type assertion

[daniel-corbett]

Thanks for the review. I have rebased and made the type assertion checks suggested by @ShimmerGlass