improve --mbps option stability and performance

[fklassen]

On fast links (i.e. 10GigE) the '--mbps' option is not performing as quickly as the '-t' option. This should be improved. Example:

# tcpreplay --preload-pcap --intf1=eth7 -t --loop 10 bosTap.pcap 
File Cache is enabled
Actual: 7916150 packets (3554177840 bytes) sent in 4.31 seconds.
Rated: 824635200.0 Bps, 6597.08 Mbps, 1836693.75 pps
Statistics for network device: eth7
    Attempted packets:         7916150
    Successful packets:        7916150
    Failed packets:            0
    Retried packets (ENOBUFS): 0
    Retried packets (EAGAIN):  0

# tcpreplay --preload-pcap --intf1=eth7 --mbps=4000 --loop 10 bosTap.pcap
File Cache is enabled
Actual: 7916150 packets (3554177840 bytes) sent in 8.99 seconds.
Rated: 395347936.0 Bps, 3162.78 Mbps, 880550.62 pps
Statistics for network device: eth7
    Attempted packets:         7916150
    Successful packets:        7916150
    Failed packets:            0
    Retried packets (ENOBUFS): 0
    Retried packets (EAGAIN):  0

[fklassen]

Starting to get better results, and seem to be able to get higher performance. Also can get capacity numbers for very short tests

tcpreplay --preload-pcap --intf1=eth7 -t --loop 10 bosTap.pcap 
sending out eth7 
File Cache is enabled
processing file: bosTap.pcap
processing file: bosTap.pcap
processing file: bosTap.pcap
processing file: bosTap.pcap
processing file: bosTap.pcap
processing file: bosTap.pcap
processing file: bosTap.pcap
processing file: bosTap.pcap
processing file: bosTap.pcap
processing file: bosTap.pcap
Actual: 7916150 packets (3554177840 bytes) sent in 4.00 seconds.
Rated: 876912868.4 Bps, 7015.30 Mbps, 1953130.68 pps
Statistics for network device: eth7
    Attempted packets:         7916150
    Successful packets:        7916150
    Failed packets:            0
    Truncated packets:         0
    Retried packets (ENOBUFS): 0
    Retried packets (EAGAIN):  0

tcpreplay --preload-pcap --intf1=eth7 --mbps=4000 --loop 10 bosTap.pcap 
sending out eth7 
File Cache is enabled
processing file: bosTap.pcap
processing file: bosTap.pcap
processing file: bosTap.pcap
processing file: bosTap.pcap
processing file: bosTap.pcap
processing file: bosTap.pcap
processing file: bosTap.pcap
processing file: bosTap.pcap
processing file: bosTap.pcap
processing file: bosTap.pcap
Actual: 7916150 packets (3554177840 bytes) sent in 7.01 seconds.
Rated: 499998852.0 Bps, 3999.99 Mbps, 1113637.55 pps
Statistics for network device: eth7
    Attempted packets:         7916150
    Successful packets:        7916150
    Failed packets:            0
    Truncated packets:         0
    Retried packets (ENOBUFS): 0
    Retried packets (EAGAIN):  0

tcpreplay --preload-pcap --intf1=eth7 --mbps=7000 bosTap.pcap 
sending out eth7 
File Cache is enabled
processing file: bosTap.pcap
Actual: 791615 packets (355417784 bytes) sent in 0.406842 seconds.
Rated: 873601506.2 Bps, 6988.81 Mbps, 1945755.35 pps
Statistics for network device: eth7
    Attempted packets:         791615
    Successful packets:        791615
    Failed packets:            0
    Truncated packets:         0
    Retried packets (ENOBUFS): 0
    Retried packets (EAGAIN):  0

tcpreplay --preload-pcap --intf1=eth7 -t -L 100 bosTap.pcap 
sending out eth7 
File Cache is enabled
processing file: bosTap.pcap
Actual: 100 packets (26670 bytes) sent in 0.000096 seconds.
Rated: 277812500.0 Bps, 2222.50 Mbps, 1041666.66 pps
Statistics for network device: eth7
    Attempted packets:         100
    Successful packets:        100
    Failed packets:            0
    Truncated packets:         0
    Retried packets (ENOBUFS): 0
    Retried packets (EAGAIN):  0