ObjectStore: add a bufferlist-based getattrs() function

Signed-off-by: Greg Farnum <greg@inktank.com>
OSD: object_copy_data_t should take advantage of bufferlist-based getattrs

Now we don't need to do the silly bufferlist-bufferptr non-magic.

Signed-off-by: Greg Farnum <greg@inktank.com>