Add setting to indicate if log won't be truncated

[urt30plus]

https://github.com/urt30plus/urt30t/blob/6c21acfe1f7ba3efa9c21a89b19c5291f406fa17/urt30t/core.py#L249

If we know the log won't be truncated we can skip os stat calls. This should be a setting that defaults to no truncate.