Compute reviewer stats

Summary:
The final goal is to display reviewers response time on homepage.
This is a building block for it.

The algorithm is quite strict - it doesn't count simple comment as response because reviewers would be able to cheat with comments like "I'm overwhelmed right now and will review next week".
We are more liberate in Phabricator where reviewers response with comments without changing the status quite often but I'm not trying to improve response times in Phabricator so this is irrelevant.
Reviewers in Facebook changes status more often (to clean their queue) so I follow this approach.

There is currently no way to track reviewers silently added and removed in Edit Revision but it's not a big deal.
The algorithm doesn't track commandeered revision, there's a TODO for it.

Response times are put in two buckets: $reviewed and $not_reviewed.
$reviewed contains reviewers who took action, $not_reviewed contains reviewers who didn't respond on time.
I will probably compute average time from $reviewed and raise it for those $not_reviewed that are higher than this average.
The idea is to not favor reviewers who were only lucky for being in a group with someone fast.

Test Plan: Passed test.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3062