Rules on how/when merge a stale PR

[avivace]

I propose the following guidelines (most of it are things we already do, but never formalised).

For disruptive PRs:

• Wait for 2 maintainers approval
• Every week of inactivity/staleness, ping maintainers
• After 5 weeks from last activity, PR is merge-able with only 1 maintaner approval

For non-disruptive PRs:

• Wait for 1 maintainer approval and merge it after 5 days
• If a maintainer wants to put a veto and to make the PR wait for his/her input after the mentioned period, he can label the issue/label as "veto"