[PATCH] Use StringJoiner where appropriate in java.base

[Сергей Цыпанов]

> Yes, StringJoiner is really good at joining Strings (hence the name).
> But it is not very good at joining primitives. Because you have to
> convert each primitive to a String before you can add it.
> StringBuilder does not need to allocate a String when you add a
> primitive.

I've reworked my patch to affect only the spots where we either already have Strings 
or do convertion to String (just like StringBuilder.append(Object) does using String::valueOf).

Also I've dropped erroneous changes to Atomic*Array (thanks Martin for spotting this).

Regards,
Sergey Tsypanov
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