Prevent sporadic hangs with Solid Cable

I can simulate hangs in the server by opening many tabs and, in parallel, perform many
modifications to a monitored file very quickly. This seems to fix those for good.

lib/hotwire/spark/installer.rb

@@ -36,7 +36,13 @@ def monitor(paths_name, action:)
     end
 
     def broadcast_reload_action(action, file_path)
-      Hotwire::Spark.cable_server.broadcast "hotwire_spark", reload_message_for(action, file_path)
+      broadcast_mutex.synchronize do
+        Hotwire::Spark.cable_server.broadcast "hotwire_spark", reload_message_for(action, file_path)
+      end
+    end
+
+    def broadcast_mutex
+      @broadcast_mutex ||= Mutex.new
     end
 
     def reload_message_for(action, file_path)