TCP session is hanging even when NETCONF keeplive did not get reply.

Steps to reproduce:

1. start ODL + start testtool
2. mount testtool to ODL (keepalive-delay=5s, default-request-timeout-millis=3000)
3. wait until connection-status is connected
4. block packets on testtool going from ODL to testtool `iptables -I INPUT -p tcp --dport 17830 -j DROP`
5. in worst case connection should be closed after 8s (5s + 3000ms) because keepalive does not get reply. However, keepalive is sent, nothing happens and connection-status is still "connected"