enhance description of CHANGELOG.md

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: count prevent cow requests in a correct manner

Multiple execution of "dog vdi snapshot" can be overlapped. The
variable should not be boolean.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep, dog: use suitable name for prevent/allow cow operations

Current request names SD_OP_PREVENT_COW and SD_OP_ALLOW_COW aren't
suitable, because they prevent inode update, not COW. This patch
renames them and change related variable names.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>