Record lines_covered and lines_uncovered stats

Summary:
This will pull in aggregate statistics at the JobStep, Job, and Build level
for lines covered and lines uncovered (which allows us to calculate an overall
percentage)

Reviewers: bryan, showell

Reviewed By: showell

Subscribers: ziga, zviad, mattr, cf

Differential Revision: https://tails.corp.dropbox.com/D48027