Cache viewer spaces

Summary:
Ref T8575. Although we cache spaces as a whole, we don't cache viewer spaces. This can still do a lot of work if they have complex policies.

Instead, cache them in the request cache.

Test Plan: Saw this account for 37% of a page in produciton (303ms on on 835ms).

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8575

Differential Revision: https://secure.phabricator.com/D13324