Implement rate limiter.
Implement rate limiter.

 * It uses hardware rate limiter when available.
   (ixgbe PMD supports it!)

 * ixgbe's rate limiter includes CRC (4 bytes) for each Ethernet frame
   when it internally counts the transmitted bytes.
   We adjust the offered throughput value to match full line rates
   including all Ethernet overheads.

 * TODO: Software rate limiter does NOT work reliably yet...

ANLAB-KAIST/pspgen-dpdk