Put a readthrough cache in front of inline context construction

Summary: Ref T13513. Inline comment context information is somewhat expensive to construct and can be cached. Add a readthrough cache on top of it.

Test Plan: Loaded a source code changeset with many inline comments, used Darkconsole to inspect query activity. Saw caches get populated. Updated cache key, saw caches regenerate. Browsed Diffusion, nothing looked broken.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21279