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• Browsed as a user with an out-of-date availability cache.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