Measure coverage using codecov

[bassosimone]

We were using coveralls. The best client for C++ seems to be cpp-coveralls, which:

1. does not work with Python 3.3, which is what we have in Ubuntu 18.04

2. does not work with Debian testing even though this issue has been fixed in master

3. even when using the master branch in Debian testing, it shows other issues that I could not easily understand or fix

At the same time, using an older distribution image is problematic because the code in here is assuming that we use OpenSSL 1.1.0+.

I spent a fair amount of time trying to fix this issue in several ways, then I decided to give up and use a much better strategy:

1. measure the coverage using lcov which is more likely to be up-to-date with gcc format than cpp-coveralls since it's developed by the GNU project

2. use codecov that allows to submit a .lcov file

3. also, use codecov because their client is written in bash, hence it does not suffer of all the issues that plague Python dependency management (long live, Go static binaries!).

As a note for the future self, it was also theoretically possible to avoid the lcov step, except that there is more than a single test binary, hence there are multiple coverage measurements. The default behaviour of codecov.bassh is such that it only takes one of these measurements, hence reporting a wrong coverage measurement (here's a place where cpp-coveralls is clearly doing better than codecov). This issue has been discussed, e.g., in JoakimSoderberg/coveralls-cmake#17. My solution to this problem is to let lcov (which should be in sync with gcc) do the dirty work of merging the various coverage reports and produce a consistent summary.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@819b8e3). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #77   +/-   ##
=========================================
  Coverage          ?   87.96%           
=========================================
  Files             ?        5           
  Lines             ?     2725           
  Branches          ?        0           
=========================================
  Hits              ?     2397           
  Misses            ?      328           
  Partials          ?        0

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