sheep, dog: rename some read/write_object helpers

- add prefix for sheep helpers
- rename sd_{read, write}_object as dog_xxx in dog

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: fix local_discard_obj

- clear bitmap for hyper valume
- return error if inode operatoin fails as before

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
http: add object allocator

This implements a per-vdi allocator for http extent-based container.

We use meta object tracks the free information of data vdi for the object
allocation in a free list. Free list is a redundant structure for bitmap for
faster allocation.

future enhancements:
- Add allocation group for scalability and solve the meta size limitation
- use aio to speed up discard of objects
- add range support for inode update

Signed-off-by: Liu Yuan <namei.unix@gmail.com>