sync: reduce chunk size to 3MB

this will increase the number of round trips but will make the sync
more reliable on slow devices that take longer to process 7MB
chunks.

Notesnook.API/Hubs/SyncV2Hub.cs

@@ -257,7 +257,7 @@ namespace Notesnook.API.Hubs
                     ids,
                     size: 100,
                     resetSync: device.IsSyncReset,
-                    maxBytes: 7 * 1024 * 1024
+                    maxBytes: 3 * 1024 * 1024
                 );
 
                 await foreach (var chunk in chunks)