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/lock-instrument_test.go

@@ -228,6 +228,25 @@ func verifyLockState(l lockStateCase, t *testing.T, testNum int) {
 	verifyLockStats(l, t, testNum)
 }
 
+func TestGetOpsID(t *testing.T) {
+	// Ensure that it returns an alphanumeric result of length 16.
+	var id = getOpsID()
+
+	if len(id) != 16 {
+		t.Fail()
+	}
+
+	var e rune
+	for _, char := range id {
+		e = rune(char)
+
+		// Ensure that it is alphanumeric, in this case, between 0-9 and A-Z.
+		if !(('0' <= e && e <= '9') || ('A' <= e && e <= 'Z')) {
+			t.Fail()
+		}
+	}
+}
+
 // TestNewDebugLockInfoPerVolumePath -  Validates the values initialized by newDebugLockInfoPerVolumePath().
 func TestNewDebugLockInfoPerVolumePath(t *testing.T) {
 	lockInfo := &debugLockInfoPerVolumePath{