Add a waker to the RPC handler (#2721)

## Issue Addressed

Attempts to fix #2701 but I doubt this is the reason behind that.

## Proposed Changes

maintain a waker in the rpc handler and call it if an event is received

beacon_node/lighthouse_network/src/rpc/handler.rs

@@ -132,6 +132,9 @@ where
     /// Fork specific info.
     fork_context: Arc<ForkContext>,
 
+    /// Waker, to be sure the handler gets polled when needed.
+    waker: Option<std::task::Waker>,
+
     /// Logger for handling RPC streams
     log: slog::Logger,
 }
@@ -227,6 +230,7 @@ where
             max_dial_negotiated: 8,
             outbound_io_error_retries: 0,
             fork_context,
+            waker: None,
             log: log.clone(),
         }
     }
@@ -431,6 +435,10 @@ where
             RPCSend::Response(inbound_id, response) => self.send_response(inbound_id, response),
             RPCSend::Shutdown(reason) => self.shutdown(Some(reason)),
         }
+        // In any case, we need the handler to process the event.
+        if let Some(waker) = &self.waker {
+            waker.wake_by_ref();
+        }
     }
 
     fn inject_dial_upgrade_error(
@@ -518,6 +526,13 @@ where
             Self::Error,
         >,
     > {
+        if let Some(waker) = &self.waker {
+            if waker.will_wake(cx.waker()) {
+                self.waker = Some(cx.waker().clone());
+            }
+        } else {
+            self.waker = Some(cx.waker().clone());
+        }
         // return any events that need to be reported
         if !self.events_out.is_empty() {
             return Poll::Ready(ProtocolsHandlerEvent::Custom(self.events_out.remove(0)));