This is a consolidation effort, avoiding usage
of naked strings in codebase. Whenever possible
use constants which can be repurposed elsewhere.
This also fixes `goconst ./...` reported issues.
`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
