Switch lagg(4) statistics to counter(9).

The lagg(4) is often used to bond high speed links, so basic per-packet +=
on statistics cause cache misses and statistics loss.

Perfect solution would be to convert ifnet(9) to counters(9), but this
requires much more work, and unfortunately ABI change, so temporarily
patch lagg(4) manually.

We store counters in the softc, and once per second push their values
to legacy ifnet counters.

Sponsored by:	Nginx, Inc.