do_sleep() needs to calculate the length of time, if any, before tcpreplay 
should send the next packet.  Packet timings can be based on any of the following:

- Mbps per sec
- Pkts per sec
- Ratio compared to capture rate
- Full speed (no sleep)

For first packet we need to do the following:
- Get start (current) time
- Get start ts from pcap file
- Send packet

For each packet thereafter:
- Get current time
- Get ts for current packet
- Determine deltas for ts of first/current and time of first/current:

For Mbps:
- TTS = packet size * 8 / Mbps
- if TTS >????
- Sleep for (TTS - (current time - last send time))

For pkts per sec:
- 1 / PPS = # of sec/usec to sleep between packets (TTS)
- Sleep for (TTS - (current time - last send time))

For Ratio:
- Determine delta between current/first packet
- TTS = ratio * delta
- Sleep for (TTS - (current time - first send time))

