rgw: share time skew check between v2 and v4 auth

rgw: share time skew check between v2 and v4 auth

this moves the new std::chrono-based v4 time skew check into a common
helper function, then uses that in place of the v2 check

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit bc98772bddab44bf60fd2b081bff58198b7c043e)

Conflicts:
     src/rgw/rgw_auth_s3.cc - avoid C++17-ism (std::chrono::abs) in
                              is_time_skew_ok()