Enable golangci lint

[mihaitodor]

Hey @relistan, this is a big-ish PR with quite a few cleanup changes that I'd like to get merged before I proceed with implementing #44. It was a good way for me to re-explore the code a bit and see what's in here. I'm happy to do adjust these changes in any way or not do them at all, but the main goal was to remove all warnings reported by golangci-lint and enable it for Travis builds.

Changes to review individually:

• 4ae802a - Go 1.12 is making goconvey panic so I updated it to latest. I had to fiddle around with getting Dep to update https://github.com/jtolds/gls as a transitive dependency.
• c69b1b9 - syscall.SIGSTOP can't be trapped or ignored, so no point trying
• 5d2c03d - Refactor deprecated http.CloseNotifier. Please have a close look at this one.
• 2ea6bbc - This looks broken in the current code. The linter has revealed the issue.
• ae4cdf0 - Log errors returned by RemoveListener - Initially, I was lazy and thought it would be simpler to not return errors at all from this function. I reverted that change and added error checks higher up the call stack based on the feedback here.
• 73272a4 - Change the state in StateChangedEvent to be a pointer type - It looks like this should have been a pointer from the beginning to avoid warnings about potentially copying locks around. I don't think I'm breaking anything by changing this struct.
• ed842f9 - Return error from service.Decode if the input can't be decoded - Refactored according to the feedback received here. Sidecar only uses this function here and haproxy-api doesn't seem to use it.
• 45322d7 - Make the --cluster-ip flag optional and check Kingpin parse errors - We weren't checking the error when parsing the Kingpin flags, so this flag was never mandatory. Looking at its usage, I don't think it should be, given that it stomps over whatever is set in the SIDECAR_SEEDS environment variable. I adjusted the Readme to explain this.
• 784edf6 - Generic code cleanup, error handling and extra logging. Shouldn't change any behaviour, but please check.
• 49a5b0c - Travis cleanup - Seems to still work, so should be fine
• b7a13df - Enable golangci-lint for Travis builds, yay!
• 4f60f24 - Bump up go to 1.12. The latest golangci-lint needs at least go 1.10, so I thought it's best to just upgrade to the latest version. I can fall back to go 1.10 if you prefer that.

[mihaitodor]

@relistan Regarding the "Change the state in StateChangedEvent to be a pointer type" change, I did some testing in our dev environment with the following setup on a test host:

• update the Sidecar dependency in haproxy-api from this branch and build a new haproxy-api container
• Start 2 haproxy-api containers: haproxy1 on port 7778 and haproxy2 on port 7779
• Run Sidecar on the host from this branch with LISTENERS_URLS="http://localhost:7778/update,http://localhost:7779/update" and --cluster-ip="a_valid_sidecar_seed"

At this point, both haproxy-api containers have the expected config written in /etc/haproxy.cfg.

As a first experiment, I bounced a service and noticed that both haproxy-api containers have received the update and started proxying the new service instance.

Then, as a second experiment, I also took down Sidecar on one of the existing nodes in the cluster and observed updates coming into both haproxy-api containers, which seemed consistent.

I didn't observe any crashes or unexpected messages, so I believe 870816f is safe. Please let me know if you wish me to do any other experiments.