pf: switch to getmicro(up)time()

It is reported that micro(up)time() performs poorly in certain
virtualisation scenarios. Absolute accuracy isn't required here, so
switch to the slightly less accurate (as per the man page) get-variants.

PR: 295043
MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit e4130ac13d2fe9c17b8f8e86d39a7b5223fe7b79)