Use SO_INCOMING_CPU with bound ports

[zwoop]

When we migrate to use SO_REUSEPORT and bind the listening ports on all ET_NET threads, we should use SO_INCOMING_CPU too. It should be optional and perhaps somehow associated with the feature we already have of CPU affinity for threads.

[bryancall]

SO_INCOMING_CPU support was added in #11366 (first released in the 10.0.0 release; in the current 10.1.2 release). It's an optional socket option (net.sock_option_flag_in) that binds to the listening thread's CPU affinity, alongside the per-thread SO_REUSEPORT path, exactly as this issue requested. Closing as done.