Summary: Ref T7707. Caches availability on users to reduce the cost of loading handles. This cache is very slightly tricky to dirty properly.
Test Plan:
- Use DarkConsole to examine queries; saw cache hits, miss+fill, dirty.
- Saw availability change correctly after canceling, joining, declining events.
- Saw no queries to Calendar for pages with only availability data.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T7707
Differential Revision: https://secure.phabricator.com/D12838
