making pf faster


pf is not exactly slow, but we thought it can be faster :)

packets travel in so-called mbufs through the system

mbufs are pre-allocated, getting them is cheap
putting them back was kinda slow... dlg/tedu fixed that :)
dlg's 10GE test box went from 700 to 900 MBit/s, just because of that: not calling microuptime() every time we put an mbuf back