Recreate #311 (#317)

## Description of Changes
Recreating #311 using a branch in this repo in the hope of fixing the
Unity build.

## API

 - [ ] This is an API breaking change to the SDK


## Requires SpacetimeDB PRs

## Testsuite

SpacetimeDB branch name: master

## Testing

- [x] blackholio
- [ ] bitcraft

---------

Co-authored-by: Guribo <[email protected]>

Author: kazimuth

src/WebSocket.cs

@@ -50,6 +50,10 @@ public WebSocket(ConnectOptions options)
         public event OpenEventHandler? OnConnect;
         public event ConnectErrorEventHandler? OnConnectError;
         public event SendErrorEventHandler? OnSendError;
+
+        /// <summary>
+        ///  Called directly by background task (not on main thread!)
+        /// </summary>
         public event MessageEventHandler? OnMessage;
         public event CloseEventHandler? OnClose;
 
@@ -337,7 +341,8 @@ await Ws.CloseAsync(WebSocketCloseStatus.MessageTooBig, closeMessage,
                     if (OnMessage != null)
                     {
                         var message = _receiveBuffer.Take(count).ToArray();
-                        dispatchQueue.Enqueue(() => OnMessage(message, startReceive));
+                        // directly invoke message handling
+                        OnMessage(message, startReceive);
                     }
                 }
                 catch (WebSocketException ex)