smp: allow disabling cpu affinity
smp: allow disabling cpu affinity

cpu affinity is bad for overprovisioned environments (e.g. docker); allow
disabling it with a new option --thread-affinity 0.

avikivity/seastar,kjniemi/seastar,scylladb/seastar,cloudius-systems/seastar,dreamsxin/seastar,syuu1228/seastar,kjniemi/seastar,avikivity/seastar,scylladb/seastar,cloudius-systems/seastar,syuu1228/seastar,kjniemi/seastar,cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,dreamsxin/seastar,dreamsxin/seastar