Added functionality for new tcp stacks.

[rrs]

Summary

Sometimes, during the development of a new tcp stack, you are going to need a trigger
to tell you the rexmit timer went off. A prime example of that is RACK and PRR, you need
to know all those things you have been carefully tracking need a reset (you loose the scoreboard
and all RACK ack flags as well as reset PRR state when a rexmit timer goes off).
This adds that functionality and also moves the existing fast path stack to use the excellent
late suggestion I received using the .tfb_tcp_output = foo, instead of the full structure.

Test Plan

I will test this with RACK since I need the timeout (and am actually already testing it :-D)