rgw: get_v4_canonical_request_hash doesn't depend on req_state anymore.

rgw: get_v4_canonical_request_hash doesn't depend on req_state anymore.

In AWSv4 the hash of real, transfered payload IS NOT necessary to form
a Canonical Request, and thus verify a Signature. x-amz-content-sha256
header lets get the information very early -- before seeing first byte
of HTTP body. As a consequence, we can decouple Signature verification
from payload's fingerprint check. Although RadosGW doesn't do that for
now, the situation will definitely change in the future.

Signed-off-by: Radoslaw Zarzynski <rzarzynski@mirantis.com>