async/dpdk: do not use worker id when creating worker

async/dpdk: do not use worker id when creating worker

so we can drop the `i` parameter in a later change.

also restructure DPDKStack::spawn_worker() to capture variables by value
instead of by reference, we cannot assume that the variables allocated on
stack are still available when the function is scheduled on another
stack and core.

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit db8d9f6641ea7280e46cd985f8a3bb17fe10808e)