Enable GitLab CI testing

Implemented GitLab CI testing process additionally to existing Travis
CI. The new testing process is added to run tests faster. It requires to
control a load of machines to avoid flaky fails on timeouts. GitLab CI
allows us to run testing on our machines.

Created 2 stages for testing and deploying packages.

The testing stage contains the following jobs that are run for all
branches:

* Debian 9 (Stretch): release/debug gcc.
* Debian 10 (Buster): release clang8 + lto.
* OSX 14 (Mojave): release.
* FreeBSD 12: release gcc.

And the following jobs that are run of long-term branches (release
branches: for now it is 1.10, 2.1 and master):

* OSX 13 (Sierra): release clang.
* OSX 14 (Mojave): release clang + lto.

The deployment stage contains the same jobs as we have in Travis CI.
They however just build tarballs and packages: don't push them to S3 and
packagecloud.

In order to run full testing on a short-term branch one can name it with
'-full-ci' suffix.

The additional manual work is needed when dependencies are changed in
.travis.mk file ('deps_debian' or 'deps_buster_clang_8' goals):

 | make GITLAB_USER=foo -f .gitlab.mk docker_bootstrap

This command pushes docker images into GitLab Registry and then they are
used in testing. Pre-built images speed up testing.

Fixes #4156