Use codecov for better code coverage reporting

Most engineering teams in Gitlab will have coverage metric OKRs for Q4 2017.

In light of this, I'm adding codecov support to the Gitaly codebase:

1. We get trends over time of whether code coverage is going up or down
2. On each merge request, we get a summary of code coverage changes
3. We get an easy way to view coverage on a per module and per file basis. See https://codecov.io/gl/gitlab-org/gitaly

Additionally, this MR fixes the Gitaly coverage task which has been taking almost half-an-hour to execute and has been failing on each test.

This has significantly sped up the total build time.