Separate out validating v/s parsing logic in
isValidLocationConstraint() into parseLocationConstraint()
and isValidLocation()
Additionally also set `X-Amz-Bucket-Region` as part of the
common headers for the clients to fallback on in-case of any
region related errors.
`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
