Replace Codecov with a reliable service

[flelli]

Codecov seemed to work well in the beginning but it has some serious drawbacks, for example:

• uploading metrics intermittently breaks (see here) so we either:
  • leave it as a required step in our pipelines (making them fail and retrying manually all the failed jobs because of it)
  • make it optional, in which case we are never sure that metrics are uploaded and updated
• metrics are often times inconsistent: for example two commits that do not change code (nor tests) result in having different coverage

For these reasons we should try some other services.

Option number one is Sonarcloud and we already have it in the backlog. Although we meant to use it for code quality checks it also provides code coverage.

Other options may be:

• coveralls
• codeclimate
• codacy
• coverity

[flelli]

Now that we have Sonarcloud (#47) we have another (primary) analysis for code coverage.

However, for the time being, we also keep Codecov as we can remove it any time.