The `helper_metadata` issue is a bit complicated. For some items (e.g.
`is_paid`) copying makes no sense. For other items (e.g. `irc_channel`)
copying does make sense. Two different flags seems a bit ugly but I think
let's go with that for now. Another option would be to clear `is_paid` if
it is seen on an outbound message directly from a conversation or routing
block? Not entirely sure which these options is better.