Sleep based on fixed point in time to avoid time drift when replaying packets

[Sup3rGeo]

Fixes #724

This is missing the respective changes in send_dual_packets and probably a test. Maybe @fklassen could fill in the last pieces?

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Create packets (1 second apart):

randpkt -t udp -c 100001 test100000.pcap

Command:

sudo tcpreplay -i lo -K --multiplier=20000 /home/xxx/test100000.pcap

Without fixes (5.26 seconds):

Actual: 100001 packets (251812433 bytes) sent in 5.26 seconds
Rated: 47836517.5 Bps, 382.69 Mbps, 18997.07 pps
Flows: 94438 flows, 17940.27 fps, 99807 flow packets, 0 non-flow
Statistics for network device: lo
    Successful packets:    100001
    Failed packets:      0
    Truncated packets:     0
    Retried packets (ENOBUFS): 0
    Retried packets (EAGAIN): 0

With fixes (5.00 seconds):

Actual: 100001 packets (251812433 bytes) sent in 5.00 seconds
Rated: 50362486.6 Bps, 402.89 Mbps, 20000.20 pps
Flows: 94438 flows, 18887.60 fps, 99807 unique flow packets, 0 unique non-flow packets
Statistics for network device: lo
    Successful packets:    100001
    Failed packets:      0
    Truncated packets:     0
    Retried packets (ENOBUFS): 0
    Retried packets (EAGAIN): 0

[fklassen]

I haven't seen this issue before. Thanks for pointing it out.

This PR appears to introduce a timestamp for every packet, which will have an incredible impact on performance. I'll shelve for now, and will see if it is possible to have a large magnifier without introducing this issue.

[ruben-TC]

@Sup3rGeo Thank you.
I tested and am now using your fix, it works great for me (replaying video data where real time sync is top priority).
I also experienced lag (for a large 10min playback it would get up to 1min out of sync), now it's replaying in real time no delay at all!

Also thank you to the team for this tool, it works very well.