[Java] Use RFC3339 formats for java.util.Date

[cbornet]

• Jackson has an ISO8601Utils class that can be used to easily make a RFC3339DateFormat (or simply use the existing ISO8601Dateformat but it loses the timezone and milliseconds when serializing).
• Gson has an equivalent UtcDateTypeAdapter For Gson, it's easy to write a TypeAdapter using gson's ISO8601Utils.

These are more performant than SimpleDateFormat and SimpleDateFormat cannot be used on android (since "XXX" pattern is not supported).