TCP: make after-idle work for transactional sessions.

The use of t_rcvtime as proxy for the last transmission
fails for transactional IO, where the client requests
data before the server can respond with a bulk transfer.

Set aside a dedicated variable to actually track the last
locally sent segment going forward.

Reported by: rrs
Reviewed by: rrs, tuexen (mentor)
Approved by: tuexen (mentor), rgrimes (mentor)
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D25016