[piotrkaminski]

Folks who have this problem might want to check out Reviewable (https://reviewable.io):

1. Discussions are automatically treated as "issues" that need to be resolved before the review is complete. Resolving can be as simple as clicking "acknowledge" without following up, but there's also a rich system of "dispositions" that lets you block a discussion, resolve it unconditionally, etc.

2. You can set multiple assignees! (Only one will be reflected in GitHub, though.)

3. You can write a custom rule for determining review completion. One of the samples is "complete only when every assignee has sent an :lgtm:", giving the assignees explicit control over merge approval.

I know you've already decided against Reviewable danpalmer, but I figured others might still be interested. :)