Tweak rate limiting point counts for omnipotent users

Summary:
Ref T13008. We haven't hit any issues with this, but I can imagine we might in the future.

When one host makes an intracluster request to another host, the $viewer ends up as the omnipotent viewer. This viewer isn't logged in, so they'll currently accumulate rate limit points at a high rate.

Instead, don't give them any points. These requests are always legitimate, and if they originated from a user request, that request should be the one getting rate limited.

Test Plan: Browsed around.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13008

Differential Revision: https://secure.phabricator.com/D18708