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/config.rs

@@ -361,6 +361,13 @@ pub enum Config {
     /// Whether automatic relay management successfully added the desired number of relays
     AutorelayFinished,
 
+    /// Sorted, space-separated relay list for which no keyupdate is due.
+    KeyupdateBaseline,
+
+    /// For tests only: keyupdate debounce window in seconds.
+    #[strum(props(default = "30"))]
+    KeyupdateDebounce,
+
     /// Whether to avoid using IMAP IDLE even if the server supports it.
     ///
     /// This is a developer option for testing "fake idle".