Migration plan:
1. Add NULLable .last_update_id column to UserPresence with default 0
for new objects.
2. Backfill the value to 0 for old UserPresences, can be done in the
background while server is running.
3. Make the column non-NULL.
4. Add new model PresenceSequence and create its rows for old realms.
Anders Kaseorg
Mateusz Mandera