This revision moves a few deck chairs around.
- We go to a global limit V_tcp_split/alloc_limits so that both rack and bbr use the same sendmap limits
- NETFLIX_STATS becomes STATS in most places except specific instances
- Until the last STATS update lands if you don't have NETFLIX_STATS defined (and you should not) STATS is undefined in bbr and rack
- The SaD experimental detection parameters is placed in the stack so that both bbr and rack can use it