On webdav harvesters, try to keep the same UUID if possible.

On webdav harvesters, try to keep the same UUID if possible on the local copy of the metadata.

To determine the UUID we are going to use the following mechanism: 
1.- Look for the file identifier on the metadata xml 
2.- If there is no file identifier, then use the name of the file 
3.- If there is a collision of uuid with existent metadata, use a random one 
4.- If we still don't have a clear UUID, use a random one (backup plan)