Update rack to the latest code used at NF.

[rrs]

Summary

There have been many changes to rack over the last couple of years, including:

a) Ability when switching stacks to have one stack query another.
b) Internal use of micro-second timers instead of ticks.
c) Many changes to pacing in forms of
   1) Improvements to Dynamic Goodput Pacing (DGP)
   2) Improvements to fixed rate paciing
   3) A new feature called hybrid pacing where the requestor can
      get a combination of DGP and fixed rate pacing with deadlines
      for delivery that can dynamically speed things up.
d) All kinds of bugs found during extensive testing and use of the
   rack stack for streaming video and in fact all data transferred
   by NF

Test Plan

Use the stack on various loads to make sure we still transfer properly, turn on BB logging and pull out key logs as well.

[rrs]

Opps missed one sysctl change that has to be in.

[gallatin]

sys/netinet/tcp_stacks/tailq_hash.c

81

Are all these includes really needed?

[rrs]

Fix rack_init() bug Gleb found, We need to look at *ptr not ptr in our null check

[rrs]

Fix another niggle from Gleb, after we seen *ptr == NULL we don't need to set
*ptr to NULL.