Identify currently running export and skip within get latest export path

[DTW-DanWard]

In case long-running export or less like event user has export running and also tries doing something that uses Get-ODUExportLatestPath (oduobject, odudiff, etc.):

1. At beginning of export create file in current export root that identifier export is running: ExportingNow.txt (or something).
2. Delete that file after successful export and post-processing complete.
3. In Get-ODUExportLatestPath, check if most recent file contains that file, if it does, skip it and check proceeding export instead.