File Move API can return 500 for directories

[jimmyca15]

When using the move api (/api/files/move) to move a directory it is possible that a 500 may occur. The cause is a race condition between the calculation of the size of the directory and the move operation.

https://github.com/Microsoft/IIS.Administration/blob/master/src/Microsoft.IIS.Administration.Files/Copy/MoveHelper.cs#L168
https://github.com/Microsoft/IIS.Administration/blob/master/src/Microsoft.IIS.Administration.Files/Copy/MoveOperation.cs#L54

Copy is not affected.