work queue: introduce dynamic roof to max nr threads

This patch add dynamic roof to max # of work threads of one particular queue,
which is proportional to the nr of nodes in the cluster. This is needed to
prevent push storm from object cache, which might otherwise saturate the cluster
with push threads.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>