make sure all datasets are closed during metadata initialization

[dromagnoli]

When investigating on a JP2ECW issue, we found (thanks to Even Rouault for that) a bug on Dataset management.

Long story short, on some metadata initialization, we acquire a Dataset but we don't close it. A deinitialization conflict between the ECW driver and GDAL Driver results into a segmentation fault as reported here:
http://trac.osgeo.org/gdal/ticket/5024

We can avoid that by properly explictly closing any Dataset.