This index is intended to optimize the performance of the very
frequently run query of "what is the presence status of all users in a
realm?".
Main changes:
- add realm_id to UserPresence
- add index for realm_id
- backfill realm_id for old rows
- change all writes to UserPresence to include
realm_id
The index is of this form:
"zerver_userpresence_realm_id_5c4ef5a9" btree (realm_id)
We will create an index on (realm_id, timestamp) in a
future commit, but I think it's a bit faster if you do
the backfill before the index.
There's also a minor tweak to the populate_db script.
Steve Howell