tcp: move cwnd and ssthresh updates into cc modules

This will pave the way of setting ssthresh differently in TCP CUBIC, according
to RFC8312 section 4.7.

No functional change, only code movement.

Submitted by: chengc_netapp.com
Reviewed by: rrs, tuexen, rscheff
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D26807