Reuse the calendar instance used during formatting

[brenuart]

See #1749

The idea is to reuse the calendar instance used for formatting using the same strategy as for DateFormat instances.
A new _calendar instance field is added to hold the calendar once initialised. However, java.text.DateFormat already has a protected calendar field that is not used. I wonder if we should use that one instead of creating another...

[cowtowncoder]

The main/only concern here is the life-cycle of StdDateFormat: is an instance created (via clone()) for serialization (in which case it is thread-safe by being only used from a single thread) or shared.
I should know it having rewritten code very recently... :-D

So I'll have to go over code again.

Another possibility, if instances were shared, would be to use AtomicReference for "leasing" instance.

[cowtowncoder]

Ok, so syncing is bit of a mess but yes, instance IS clone()s for serialization purposes so this should be safe without further complications.
I think it'll be worth further refactoring for 3.0 -- I am thinking of actually splitting read/write aspects, maybe? -- but for now this'll work.