sheepdog proto: change type of offset in object request

QEMU issues offset as a value of 64bit length.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
sheepdog proto: add a new member working_vid to sd_req

It is required for discard operation.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
sheep: use vid passed by client in discard operation

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
sheep: don't call sd_remove_object() directly in discard operation

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
tests/functional: add a new test for coexisting of object reclaim and discard

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
sheep: enforce reload when a client issues discard to snapshot

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>