pf: Add per-rule timestamps for rule and eth_rule

Similar to ipfw rule timestamps, these timestamps internally are
uint32_t snaps of the system time in seconds. The timestamp is CPU local
and updated each time a rule or a state associated with a rule or state
is matched.

Reviewed by: kp
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D34970