fix golint issues in core/stores (#527)

core/stores/mongo/util.go

@@ -4,6 +4,7 @@ import "strings"
 
 const mongoAddrSep = ","
 
+// FormatAddr formats mongo hosts to a string.
 func FormatAddr(hosts []string) string {
 	return strings.Join(hosts, mongoAddrSep)
 }