sheep/http: store accounts and containers into hyper volume for object-storage

Using hyper volume (size up to 16PB) to store large number of accounts
and containers.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep/http: add support for big object which is larger than SD_DATA_OBJ_SIZE

Using hyper volume and extent structure of onode to store large number of
objects size of which exceed SD_DATA_OBJ_SIZE.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep/http: add lock to protect container and object

Add locks to protect containers and objects when users create/delete
containers or create/delete objects in the same time.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep/http: check object beofre create or delete it

We add kv_find_object() to check whether the object is exists or not,
so it can decide how to update or delete it.

BTW, we discard data obj after deleting kv_onode.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>