For newly created MRFs, calling MRFBand::SetStatistics(...) only affects the aux metadata, not the .mrf file

[lucianpls]

When using the gdal Band->SetStatistics, only the aux metadata is affected, which is stored in the aux.xml sidecar file. MRF has support for min and max, but they are not updated. They are stored in the .mrf when copying an existing dataset.
To fix, the SetStatistics should be overloaded in MRFBand. For newly created files, since the gdal .mrf xml file is written as soon as some data needs to be written to the data file, but SetStatistics is usually called after most of the data was written, which would require re-creating the .mrf file on dataset close.