Historically we would backdate the SN change messages, but since adopting
non-blocking SN changes long ago, they're already sorted properly by creation
time, so backdating has been unnecessary for a while.
I also audited that all other error messages are saved directly after creation.
I applied deprecation attributes as appropriate as I audited.
There is no change in functionality in this commit.
Apart from clarifying what the timestamp means (it's the timestamp of the
*sender*), this intentionally breaks all the call sites, so I could have a sane
way to thoroughly audit wherever we're passing in timestamps, to see where
we're depending on them to affect sort order.
For the sake of a cleaner diff of meaningful changes, instead of "fixing"
everything in this commit, I've just added comments and renamed signatures.
