Notify superseded plugin processes and support SDK draining

sdk/tests/ui/superseded.rs

@@ -0,0 +1,11 @@
+struct Supersedes;
+#[sausage_plugin::rpc]
+impl Supersedes {
+    /// First superseded hook.
+    #[sausage_plugin::on_superseded]
+    fn a(&self) -> sausage_plugin::Result<()> { Ok(()) }
+    /// Second superseded hook.
+    #[sausage_plugin::on_superseded]
+    fn b(&self) -> sausage_plugin::Result<()> { Ok(()) }
+}
+fn main() {}