Use cases:

1/ By default everything works as before, the setting defaults to:

 $CFG->file_storage_recovery_classes = array('core_file_storage_trash');

2/ some sites may not want to delete files from $CFG->filedir at all (this helps file-less backups, filedir may be shared by independent moodle instances)

 $CFG->file_storage_recovery_classes = array();

3/ this might be used for continuous external backups

 $CFG->file_storage_recovery_classes = array('core_file_storage_backup');

 $CFG->core_file_storage_backup_filedir = '/some/masterfiledir';

4/ somebody might want to skip the trashdir when deleting files

 $CFG->file_storage_recovery_classes = array('core_file_storage_notrash');

5/ in theory this might be used on cluster nodes where normal $CFG->filedir could be used as local cache only (this is not the primary purpose of this patch)

6/ this refactoring should also help if anybody decides to reimplement file_storage and stored_file because now it does not have any trashdir logic that might collide with other file storage designs