Cache viewer spaces
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).
Reviewed By: btrahan
Maniphest Tasks: T8575
Differential Revision: https://secure.phabricator.com/D13324