memif: multi-queues support

memif: multi-queues support

- Add rx-queues and tx-queues option to the create memif CLI
- Add vlib_worker_thread_barrier_sync () to memif_conn_fd_read_ready () as
the latter function may disconnect the ring and clean up the shared memory.
- On transmit, write the rid (queue number) to the socket.
- On receive, read the rid and trigger the interrupt for the corresponding
thread.

Change-Id: If1c7e26c7124174678f047909cbc33e931eaac8c
Signed-off-by: Steven <sluong@cisco.com>