Mitigate a case where TCP rack can send an extra ack.

Rack will in theory send an extra rate limited ack when we get to a closing state (sending a FIN) so that
if we have only 1 packet outstanding we might encourage the connection to close out. However it does this
always which is not always wise. Change it so that it only does that if its been more than an srtt since
we have had some activity i.e. a send or a receive of a packet.
Reviewed by:tuexen, rscheff
Differential Revision:https://reviews.freebsd.org/D55459