DAOS S3 Mapping

Use libdfs to maintain compatibility with POSIX namespace (lingua franca).

S3 Bucket = DAOS Container

Bucket metadata can be stored in container attributes (https://github.com/daos-stack/daos/blob/master/src/include/daos_cont.h#L436)

S3 Object = DAOS FS File (DFS)

Object metadata can be stored in extended attributes (https://github.com/daos-stack/daos/blob/master/src/include/daos_fs.h)