Stop two special cache writes in read-only mode
Summary:
Ref T10769. The user availability cache write shouldn't happen in read-only mode, nor should the Differential parse cache write.
(We might want to turn off the availbility feature completely since it's potentially expensive if we can't cache it, but I think we're OK for now.)
Test Plan:
In read-only mode:
- Browsed as a user with an out-of-date availability cache.
- Loaded an older revision without cached parse data.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T10769
Differential Revision: https://secure.phabricator.com/D15988