sheep: handle a case of duplicated create and write requests for iSCSI multipath

In a case of iSCSI multipath, there is a possibility of duplicated
create and write requests from different tgtd. In this case, existing
objects can be overwritten by the duplicated requests. This patch
removes this overwite problem by let sheep check existence of newly
created object.

Cc: YAMADA Hideki <yamada.hideki@lab.ntt.co.jp>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>