There was a deadlock between QgsOgrProvider's loadMetadata() and open() functions:
- loadMetadata() would first lock data source related mutex (QgsOgrProviderUtils::DatasetWithLayers::mutex)
  and then lock the global mutex (sGlobalMutex)
- the open() function would call QgsOgrProviderUtils::getLayer() where the order of locking is reversed,
  i.e. first lock the global mutex, then the data source related mutex

Fixed by updating loadMetadata() to lock data source related mutex for the minimum time
necessary (certainly before the global mutex gets locked).

Until now, creation of GDAL datasets from within OGR provider would get
serialized, because QgsOgrProviderUtils::getLayer() starts by locking
a global mutex. This is now changed to lock only for the time while
shared data structures need to be accessed, and therefore it is possible
that multiple threads can open GDAL datasets in parallel.