Editor hangs for several seconds when closing multiple asset editors

CL 51680020 added an async write in SStandaloneAssetEditorToolkitHost::SaveTabLocation(). This can cause multi-threaded write conflicts on EditorPerProjectUserSettings.ini.
As a result, the file move fails repeatedly and the editor freezes for a while.

1. Create a project from any template, or use an existing one.
2. Close the editor
3. Inflate Saved/Config/WindowsEditor/EditorPerProjectUserSettings.ini to about 30 MB to reproduce it easily. (For example, add a dummy section with about 200,000 Key=Value lines)
4. Launch the editor.
5. Open about 10 saved assets,
6. Close all asset editor windows at the same time.

Result:

The editor freezes for multiple seconds. During the freeze the log repeatedly shows:

LogFileManager: Warning: MoveFile was unable to move '.../Saved/EditorPerProjectUserSettings<GUID>.tmp' to '.../Saved/Config/WindowsEditor/EditorPerProjectUserSettings.ini' (Error Code 183), retrying in .5s...