Invalid USC DTLS channel after UscAgentUdp is terminated

Steps to reproduce:
1. Start ODL controller and install USC featrues on host1
2. Start UscAgentUdp and EchoServerUdp on host2
3. Create a USC DTLS channel
4. Send a test message to EchoServerUdp
5. Check if the message is sent back
6. Remove the channel
7. Terminate UscAgentUdp and EchoServerUdp
8. Redo from step 2 to step 4
9. The test message is not sent back because the DTLS chanel is invalid