Rewrite `StdDateFormat` ISO-8601 handling functionality

[cowtowncoder]

StdDateFormat is pretty fragile implementation, but especially so its ISO-8601 decoder part.
Problems range from "interesting" choice of valid values to concurrency/performance problems (that is, how to balance access to SimpleDateFormat). But looking at SimpleDateFormat and Calendar, real calculations in latter, not former; plus, we don't really need flexibility of SimpleDateFormat here (although for RFC1123 we may still want it).

And it probably also makes sense to rewrite ISO-8601 serialization parts while we're at it.