sheep: reload config after successful migration

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: migrate format from v3 to v4

The commit 79706e07a068 introduced incompatibility of store format
(store ec index in path instead of xattr). This patch produces
migration function for this change.

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