`principalId` i.e user identity is kept as AccessKey in
accordance with S3 spec.
Additionally responseElements{} are added starting with
`x-amz-request-id` is a hexadecimal of the event time itself in nanosecs.
`x-minio-origin-server` - points to the server generating the event.
Fixes#3556
Current master has a regression 'mc policy <policy-type> alias/bucket/prefix'
does not work anymore, due to the way new minio-go changes do json marshalling.
This led to a regression on server side when a ``prefix`` is provided
policy is rejected as malformed from th server which is not the case with
AWS S3.
This patch uses the new ``minio-go/pkg/set`` package to address the
unmarshalling problems.
Fixes#2503
