pf: add hybrid 32- an 64- bit counters

Numerous counters got migrated from straight uint64_t to the counter(9)
API. Unfortunately the implementation comes with a significiant
performance hit on some platforms and cannot be easily fixed.

Work around the problem by implementing a pf-specific variant.

Reviewed by: kp
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit defdcdd5648dc1ea789bc54bb45108fcab546a6b)