We want to err on the side of server_timestamp being
old, since we may eventually use this to make responses
just include incremental changes, and we don't want a
time window (however small) when we miss presence rows.
The clients will be able to deal with duplicate data
to the extent that the time windows are overlapping.
Also, extracting the other local var here
(for `presences`) will set up a subsequent commit
where we re-format the data for clients with
slim_presence=True.
8a1fb2dcd6