pf: implement adaptive mode

Use atomic counters to ensure that we correctly track the number of half
open states and syncookie responses in-flight.
This determines if we activate or deactivate syncookies in adaptive
mode.

MFC after: 1 week
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D32134

(cherry picked from commit bf8637181a2bb81206ff8c685f1632d07b8feb13)