pf: sctp heartbeats confirm a connection

When we create a new state for multihomed sctp connections (i.e.
based on INIT/INIT_ACK or ASCONF parameters) the new connection will
never see a COOKIE/COOKIE_ACK exchange. We should consider HEARTBEAT_ACK
to be a confirmation that the connection is established.

This ensures that such connections do not time out earlier than
expected.

MFC after: 1 week
Sponsored by: Orange Business Services