Use StringBuilder instead of StringBuffer when no concurrent access

Replacement of #60
@mattbishop No, it appends an int: https://docs.oracle.com/javase/7/docs/api/java/lang/StringBuilder.html#append(int)

It looks to me that `StringBuilder.append(String.valueOf(int))` and `StringBuilder.append(int)` do basically the same thing based on `Integer.getChars`, except that the former is quite ugly.