epoch
This function will be called every time a conversation's epoch changes.
The epoch parameter is the new epoch.
This function must not block! Foreign implementors of this inteface can
spawn a task indirecting the notification, or (unblocking) send the notification
on some kind of channel, or anything else, as long as the operation completes
quickly.
Though the signature includes an error type, that error is only present because it is required by uniffi in order to handle panics. This function should suppress and ignore internal errors instead of propagating them, to the maximum extent possible.