Fix lost forward extremities with federation

Old leaf ids were removed, but none were inserted, leading to creating
events with no previous events.

src/server_server.rs

@@ -1002,6 +1002,9 @@ fn handle_incoming_pdu<'a>(
             }
         }
 
+        // Add incoming event as forward extremity
+        extremities.insert(incoming_pdu.event_id.clone());
+
         // 12. Ensure that the state is derived from the previous current state (i.e. we calculated
         //     by doing state res where one of the inputs was a previously trusted set of state,
         //     don't just trust a set of state we got from a remote).