Allow dfs.datanode.shared.file.descriptor.path to contain multiple entries and fall back when needed

In SharedFileDescriptorFactory, it would be nice if dfs.datanode.shared.file.descriptor.path could contain multiple entries and allow fallback between them. That way, if a user doesn't have /dev/shm configured (the current default), the user could use /tmp. This is mainly a concern on BSDs and some other systems where /dev/shm doesn't exist.