Adds a `calendar` area to the local store. It syncs calendars, participant
identities, events and tasks as masters, through CalendarEvent/changes,
and answers Calendar/get, CalendarEvent/query, /get and /set from SQLite
when the server can't be reached.
- A range query gets series in both client modes, with their overrides,
and the calendar store expands them as it does for an older server.
- The synthetic-id probe gets the verdict from the last sync, so a client
started offline behaves the same once the server is back.
- Create, edit, delete and RSVP queue in `calendar_pending`, one merged
row per event, and go out through `calendar.flush` or the next sync.
A create finds itself by uid after a lost response.
- A write the server refuses, or one on a value the server changed, goes
to `calendar.conflicts`.
- `lib/store/calendar/reminders.ts` lists upcoming alerts with keys that
don't depend on synthetic ids.
Linus Rath