Add an availability cache for users

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