feat: introduce keyupdate messages informing contacts about relay changes

When the published relay list changes, key-contacts are informed with an
unsigned message carrying the re-signed key, encrypted to a chunk of contacts
at a time. It is shaped like a receipt notification naming no message, so that
cores which know nothing about keyupdates trash it as well.

See the src/keyupdate.rs module docs for the design.

src/smtp.rs

@@ -327,14 +327,15 @@ pub(crate) async fn smtp_send(
     status
 }
 
-/// Inserts a rendered message into the `smtp` table for sending.
+/// Inserts a tombstone for `rfc724_mid`
+/// and queues the rendered message for SMTP sending.
 pub(crate) async fn insert_into_smtp(
     context: &Context,
     rfc724_mid: &str,
     recipients: &str,
     rendered_message: String,
-    msg_id: MsgId,
-) -> Result<(), Error> {
+) -> Result<()> {
+    let msg_id = message::insert_tombstone(context, rfc724_mid).await?;
     context
         .sql
         .execute(