Performance problem on import of very large folders

When the importer scans folders with a very large number of files, there is a performance problem due to a quadratic algorithm.

This gets even worse if the imported files are sitting on relatively slow storage like a NAS.

The culprit is org.nuxeo.ecm.platform.importer.source.FileWithIndividualMetadasSourceNode.getChildren and its call to getPropertyTargetKey which does a quadratic scan of the listFiles.