SCTP control message retransmission

This enhancement was added to better support retransmission. A retransmitted cookie-echo/cookie-ack can pass after an association is established on the firewall, but it cannot pass after the configured handshake timeout. (For example, if a handshake timeout is configured for 20 seconds, the packets can pass through the 4-way handshake process only within the configured 20 seconds.) When the association is shut down completely, there are 2 seconds for packet (shutdown, shutdown-ack, shutdown-complete) retransmission.