sheep: rename bs_thread_request_done() -> worker_thread_request_done()

The prefix "bs_" of the name bs_thread_request_done() would've come
from the naming scheme of tgt (the "backing store"). This prefix is
unclear so this patch rename it.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>