StdDateFormat: performance improvement of '_parseAsISO8601(...)'

[brenuart]

Following the same rational as in #1749, the calendar instance used during parsing in parseAsISO8601(...) could be reused across invocations.

What you think ?

[cowtowncoder]

I think that might be a good idea. Looking at deserialization side I think DateFormat is actually cloned as well (first I thought it wasn't, but DeserializationContext.getDateFormat() does actually clone it), so reuse should be possible.
Still, I am almost tempted to attempt writing multi-threaded test just in case ... since failures wrt non-synchronized shared resources can be pretty nasty.

But there's bit of choreography still, wrt ensuring correct timezone and leniency settings are used (and perhaps Locale -- not 100% if it has any effect, but it is set on GregorianCalendar).

So, +1 for this.