Lock around heartbeat handler (#1264)

src/Microsoft.AspNetCore.Sockets/DefaultConnectionContext.cs

@@ -74,17 +74,23 @@ namespace Microsoft.AspNetCore.Sockets
         public TransferMode TransferMode { get; set; }
 
         public void OnHeartbeat(Action<object> action, object state)
+        {
+            lock (_heartbeatHandlers)
             {
                 _heartbeatHandlers.Add((action, state));
             }
+        }
 
         public void TickHeartbeat()
+        {
+            lock (_heartbeatHandlers)
             {
                 foreach (var (handler, state) in _heartbeatHandlers)
                 {
                     handler(state);
                 }
             }
+        }
 
         public async Task DisposeAsync()
         {