Add Debian Buster (10) and update "*stable" aliases

See https://lists.debian.org/debian-announce/2017/msg00003.html for the
official release announcement for Debian Stretch.

database/namespace_mapping.go

@@ -21,13 +21,15 @@ var DebianReleasesMapping = map[string]string{
 	"wheezy":  "7",
 	"jessie":  "8",
 	"stretch": "9",
+	"buster":  "10",
 	"sid":     "unstable",
 
 	// Class names
-	"oldstable": "7",
-	"stable":    "8",
-	"testing":   "9",
-	"unstable":  "unstable",
+	"oldoldstable": "7",
+	"oldstable":    "8",
+	"stable":       "9",
+	"testing":      "10",
+	"unstable":     "unstable",
 }
 
 // UbuntuReleasesMapping translates Ubuntu code names to version numbers