add threshold for simulate Application level Latency

[kylinsoong]

add threshold for simulate Application level Latency that can generate

• TCP Window Full

• TCP ZeroWindow

[kylinsoong]

With this commit, start the recv append a -L flag

ttcp -r -l 512 -L

which this will cause the recv receive speed around 0.15 MB per seconds, so the trans side should also send a little size data, like 5MB, 10MB, eg:

ttcp -n 10 -t <IP>

This most likely will simulate TCP Window Full, TCP ZeroWindow, TCP Retransmission, and TCP Dup Ack in wireshark reports.