api: add event channel overflow event

src/events/payload.rs

@@ -315,4 +315,10 @@ pub enum EventType {
     /// Event for using in tests, e.g. as a fence between normally generated events.
     #[cfg(test)]
     Test,
+
+    /// Inform than some events have been skipped due to event channel overflow.
+    EventChannelOverflow {
+        /// Number of events skipped.
+        n: u64,
+    },
 }