event: Enhance event message struct to provide origin server. (#3557)

`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

cmd/globals.go

@@ -73,6 +73,7 @@ var (
 
 	// Cache expiry.
 	globalCacheExpiry = objcache.DefaultExpiry
+
 	// Minio local server address (in `host:port` format)
 	globalMinioAddr = ""
 	// Minio default port, can be changed through command line.
@@ -80,6 +81,9 @@ var (
 	// Holds the host that was passed using --address
 	globalMinioHost = ""
 
+	// Holds the list of API endpoints for a given server.
+	globalAPIEndpoints = []string{}
+
 	// Peer communication struct
 	globalS3Peers = s3Peers{}