Avoid leveldb check for xattr when possible

Maintain a intern xattr called "spill_out" that indicates whether spilling
over into DBObjectMap. If not present, assume this is old style object and
look in DBObjectMap. Then if no xattrs in DBObjectMap, this xattr(spill_out)
also will be set to indicate.

implement #7059

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
fix up omap/xattr thing

- smaller buffers
- fix setattrs
- cleaned up rmattrs flow a bit