This splits the CI process into 3 stages and multiple jobs.

It also adds a new base image with all the dependencies pre-installed.

Stage "build" build the main project.

Stage "build_test" builds the tests. This was separated out to avoid the
build stage taking more than 1 hour on cold cache.

Stage "test" spawns four jobs and runs unit tests and functional tests
in pre-upgrade and post-upgrade states.