tcp: Preparation for allowing hardware TLS to be able to kick a tcp connection that is retransmitting too much out of hardware and back to software.

Hardware TLS is now supported in some interface cards and it works well. Except that
when we have connections that retransmit a lot we get into trouble with all the retransmits.
This prep step makes way for change that Drew will be making so that we can "kick out" a
session from hardware TLS.

Reviewed by: tuexen, gallatin
Sponsored by: Netflix Inc
Differential Revision: https://reviews.freebsd.org/D30895

(cherry picked from commit 9e4d9e4c4d79db58740a05d67645351e640aa32c)