Interop with strongswan using one connection and Child REKEY.

West is Strongswan the initiator and east, Libreswan is the responder.
west rekey the Child SA using CREATE_CHILD_SA exchange.
The libreswan is the responder for RFC 7296 #1.3.3.

Strongswan IPsec/Child SA rekey is triggered on byte count; ~7 pings.
