Enhancement mbps option accuracy

Resubmitting pull request to fine tune performance.

This enhancement appears to have most significant improvements on > 1GigE. I am testing on a 10GigE network. Performance is improved by avoiding calls to gettimeofday() when not needed, and switching from floating-point to integer math during time-critical periods. Minor performance improvements found in various routines that are called during time-critical loop.

In the following examples, the first test is using code from current synfinatic/tcpreplay master and the second is this enhancement. The pcap file contains real Internet traffic with an average packet size of 550.

Full speed, PF_PACKET injector:
```
# bin4.0_master/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

# bin4.0/tcpreplay --preload-pcap --intf1=eth7 -t --loop 10 bosTap.pcap 
File Cache is enabled
Actual: 7916150 packets (3554177840 bytes) sent in 4.15 seconds.
Rated: 856428416.0 Bps, 6851.43 Mbps, 1907506.00 pps
Statistics for network device: eth7
	Attempted packets:         7916150
	Successful packets:        7916150
	Failed packets:            0
	Retried packets (ENOBUFS): 0
	Retried packets (EAGAIN):  0
```

Using --mbps=0 to achieve full speed:
```
# bin4.0_master/tcpreplay --preload-pcap --intf1=eth7 --mbps=0 --loop 10 bosTap.pcap
File Cache is enabled
Actual: 7916150 packets (3554177840 bytes) sent in 5.15 seconds.
Rated: 690131648.0 Bps, 5521.05 Mbps, 1537116.50 pps
Statistics for network device: eth7
	Attempted packets:         7916150
	Successful packets:        7916150
	Failed packets:            0
	Retried packets (ENOBUFS): 0
	Retried packets (EAGAIN):  0

# bin4.0/tcpreplay --preload-pcap --intf1=eth7 --mbps=0 --loop 10 bosTap.pcap
File Cache is enabled
Actual: 7916150 packets (3554177840 bytes) sent in 4.15 seconds.
Rated: 856428416.0 Bps, 6851.43 Mbps, 1907506.00 pps
Statistics for network device: eth7
	Attempted packets:         7916150
	Successful packets:        7916150
	Failed packets:            0
	Retried packets (ENOBUFS): 0
	Retried packets (EAGAIN):  0

```

2Gbps:
```
# bin4.0_master/tcpreplay --preload-pcap --intf1=eth7 --mbps=2000 --loop 10 bosTap.pcap
File Cache is enabled
Actual: 7916150 packets (3554177840 bytes) sent in 15.34 seconds.
Rated: 231693472.0 Bps, 1853.55 Mbps, 516046.28 pps
Statistics for network device: eth7
	Attempted packets:         7916150
	Successful packets:        7916150
	Failed packets:            0
	Retried packets (ENOBUFS): 0
	Retried packets (EAGAIN):  0

# bin4.0/tcpreplay --preload-pcap --intf1=eth7 --mbps=2000 --loop 10 bosTap.pcap
File Cache is enabled
Actual: 7916150 packets (3554177840 bytes) sent in 14.21 seconds.
Rated: 250118080.0 Bps, 2000.94 Mbps, 557083.06 pps
Statistics for network device: eth7
	Attempted packets:         7916150
	Successful packets:        7916150
	Failed packets:            0
	Retried packets (ENOBUFS): 0
	Retried packets (EAGAIN):  0
```

4Gbps
```
# bin4.0_master/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

# bin4.0/tcpreplay --preload-pcap --intf1=eth7 --mbps=4000 --loop 10 bosTap.pcap
File Cache is enabled
Actual: 7916150 packets (3554177840 bytes) sent in 7.10 seconds.
Rated: 500588416.0 Bps, 4004.71 Mbps, 1114950.75 pps
Statistics for network device: eth7
	Attempted packets:         7916150
	Successful packets:        7916150
	Failed packets:            0
	Retried packets (ENOBUFS): 0
	Retried packets (EAGAIN):  0
```

6Gbps
```
# bin4.0_master/tcpreplay --preload-pcap --intf1=eth7 --mbps=6000 --loop 10 bosTap.pcap
File Cache is enabled
Actual: 7916150 packets (3554177840 bytes) sent in 7.05 seconds.
Rated: 504138688.0 Bps, 4033.11 Mbps, 1122858.12 pps
Statistics for network device: eth7
	Attempted packets:         7916150
	Successful packets:        7916150
	Failed packets:            0
	Retried packets (ENOBUFS): 0
	Retried packets (EAGAIN):  0

# bin4.0/tcpreplay --preload-pcap --intf1=eth7 --mbps=6000 --loop 10 bosTap.pcap
File Cache is enabled
Actual: 7916150 packets (3554177840 bytes) sent in 4.73 seconds.
Rated: 751411776.0 Bps, 6011.29 Mbps, 1673604.62 pps
Statistics for network device: eth7
	Attempted packets:         7916150
	Successful packets:        7916150
	Failed packets:            0
	Retried packets (ENOBUFS): 0
	Retried packets (EAGAIN):  0
```

6.5Gbps
```
# bin4.0_master/tcpreplay --preload-pcap --intf1=eth7 --mbps=6500 --loop 10 bosTap.pcap
File Cache is enabled
Actual: 7916150 packets (3554177840 bytes) sent in 6.94 seconds.
Rated: 512129376.0 Bps, 4097.04 Mbps, 1140655.62 pps
Statistics for network device: eth7
	Attempted packets:         7916150
	Successful packets:        7916150
	Failed packets:            0
	Retried packets (ENOBUFS): 0
	Retried packets (EAGAIN):  0

# bin4.0/tcpreplay --preload-pcap --intf1=eth7 --mbps=6500 --loop 10 bosTap.pcap
File Cache is enabled
Actual: 7916150 packets (3554177840 bytes) sent in 4.37 seconds.
Rated: 813313024.0 Bps, 6506.50 Mbps, 1811476.00 pps
Statistics for network device: eth7
	Attempted packets:         7916150
	Successful packets:        7916150
	Failed packets:            0
	Retried packets (ENOBUFS): 0
	Retried packets (EAGAIN):  0
```