Fix inefficient queries on message expiration

[jmcarp]

Currently, the message expiration task loads all messages into memory and issues a separate delete for each expired message. We should use a single delete query instead--something like

delete from messages where created_at + interval '1 hour' * hours > now()

That query won't work for all sql flavors. One option here would be to restrict to postgres. If that's a problem, then we could store the expiration time in the messages table instead of the duration to get a portable query like

delete from messages where expires_at > now()

WDYT?

[amoose]

👍
I don't see an issue with restricting to postgres in order to improve overall performance.