Fixed interfaces of rados_striper_append/write/write_full to comply with the firefly API : return 0 in case of success rather than the number of bytes written

Signed-off-by: Sebastien Ponce <sebastien.ponce@cern.ch>
Polished naming in comments and some APIs so that objects are always called objects and no files

Signed-off-by: Sebastien Ponce <sebastien.ponce@cern.ch>
Fixed include of buffer.h where the internal form was used

Signed-off-by: Sebastien Ponce <sebastien.ponce@cern.ch>
Dropped usage of ceph_file_layout in the API in favor of dedicated method for the 3 parameters that we support (stripe_unit, stripe_count and object_size)

Signed-off-by: Sebastien Ponce <sebastien.ponce@cern.ch>