NAPI ID based worker thread selection
  -N <num_napi_ids>  | --napi_ids=<num_napi_ids>

By default memcached assigns connections to worker threads in a round-robin
manner. NAPI ID based worker thread selection enables worker threads to be
selected based on the NIC HW RX queue on which the incoming request is
received.

This is enabled using SO_INCOMING_NAPI_ID socket option that is supported
in linux kernels 4.12 or higher. This socket option returns a system level
unique ID called NAPI ID that is associated with a RX queue on which the
last packet associated with that socket is received.

This allows memcached to split the incoming flows among threads based on
the RX queue on which they are received. Each worker thread is associated
with a NIC HW receive queue and services all the connection requests
received on a specific RX queue. This mapping between a memcached thread
and a HW NIC queue streamlines the flow of data from the NIC to the
application. In addition, an optimal path with reduced context switches is
possible, if epoll based busy polling
(sysctl -w net.core.busy_poll = <non-zero value>) is also enabled.

This feature is enabled via a new command line parameter -N <num> or
"--napi_ids=<num>", where <num> is the number of available/assigned NIC
hardware RX queues through which requests associated with a connection are
received. The number of napi_ids specified cannot be greater than the number
of worker threads specified using -t/--threads option. If the option is
not specified, or the conditions not met, the code defaults to round robin
thread selection.

During a normal run, each worker thread gets associated with a napi_id and
this will establish a 1:1 mapping between the thread and queues. If a new
napi_id is received after each thread is associated with its own napi_id
(this can happen if num_napi_ids argument doesn't exactly match with the
number of RX queues OR if the NIC driver goes through a reload), a stats
error counter called 'unexpected_napi_ids' is incremented and all the
napi_id's associated with the threads are reset.

If -N option is used, but the connection requests are received from a
virtual interface like loopback, napi_id returned can be 0. This condition
is tracked via a stats counter called 'round_robin_fallback'.
