[T2418] Refactor http api (and bring TLS)

[G1gg1L3s]

This PR rewrites the existing acra's HTTP API to use gin, instead of using custom http server.

It also allows to use TLS with HTTP API, which was primary goal of this PR. Ideally, it should be two PR: one with refactoring and the other with TLS. But we have what we have ¯_(ツ)_/¯.

To maintain backward compatibility, by default the server doesn't use the TLS. To enable it, the user should provide certificates and keys, as well as --http_api_use_tls flag. Without it, the acra will produce a warning that the API is used without encryption. We can change this flag to true later and deprecate insecure usage of the API.

To move the server to the gin, I've studied the translator code, so many elements are familiar.

The only issue that is still present is the graceful shutdown. As it turns out, it is not that graceful, even in the translator, and I don't know whether it's something intended or wrong.

Checklist

• [x] Change is covered by automated tests
• [x] The coding guidelines are followed
• [x] Public API has proper documentation in the Acra documentation site or has PR on documentation repository with new changes
• [x] ~CHANGELOG.md is updated (in case of notable or breaking changes)~
• [x] CHANGELOG_DEV.md is updated
• [x] ~Benchmark results are attached (if applicable)~
• [x] ~Example projects and code samples are up-to-date (in case of API changes)~

[G1gg1L3s]

@Lagovas, please take a loot at the last commit and tell if it's correct or if it's not going to work in some cases. Before that, the restart with SIGHUP didn't work, because goroutines weren't able to stop during timeout. Right now it works and successfully serves the http api requests (though I haven't checked it with the TLS). However, the following log is still printed:

level=error msg="Handling HTTP API requests" connection_string="tcp://0.0.0.0:9090/" error="accept tcp [::]:9090: use of closed network connection" from_descriptor=false

Edit: checked, as expected, SIGHUP doesn't work with TLS: the server is restarted, but cannot serve TLS requests.