lb: backend connections hang in TIME_WAIT state after closing

[vdmit11]

Scenario:

1. start a backend server (e.g. nc -lp 8080)
2. start Tempesta FW
3. wait until the connection is established
4. stop the backend server
5. wait for fin_timeout (/proc/sys/net/ipv4/tcp_fin_timeout)
6. check that the TCP connection is released (netstat -an | grep TIME_WAIT)

Got:
tcp 0 0 127.0.0.1:23112 127.0.0.1:8080 TIME_WAIT
The connection TCP hangs in the TIME_WAIT state even when the tcp_fin_timeout is expired.

Expected:
The closed TCP connection should be released immediately without handing in the TIME_WAIT state.
Or at least, it should be released after tcp_fin_timeout.

The TIME_WAIT means that either FIN is not received or it is not handled correclty.
Since we run everything locally, the FIN is likely received, so likely it is something wrong with FIN handling.

[krizhanovsky]

Reassign back to Dmitry - need to recheck the issue when #46 is fixed.

[keshonok]

It's been a long time and lots of code changes had been made.
Unable to reproduce now.