MINOR: payload: add new direction-explicit sample fetches

MINOR: payload: add new direction-explicit sample fetches

Similarly to previous commit fixing "hdr" and "cookie" in HTTP, we have to deal
with "payload" and "payload_lv" which are request-only for ACLs and req/resp for
sample fetches depending on the context, and to a less extent with other req_*
and rep_*/rep_* fetches. So let's add explicit "req." and "res." variants and
make the ACLs rely on that instead.