Right now history is lost on bouncer restart (for clients that don't support chathistory). History is stored in an in-memory ring buffer.
This is an issue when upgrading soju: users will potentially miss messages.
We can switch to using the on-disk logs. We also need to store a pointer to the last sent log entry for each client.
Some WIP version of this is available in the
Done in 1e4ff4947246 ("Save delivery receipts in DB").