R1:1970ceefe3d4

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…