RFR: Differentiate young generation marking. [v3]

[Roman Kennke]

On Wed, 9 Dec 2020 10:00:19 GMT, Bernd Mathiske <bmathiske at openjdk.org> wrote:

>> This change is large to achieve uninterrupted functionality for single-generational Shenandoah while planting code that differentiates two distinct generations and concurrent marking for the young generation.
>> 
>> Where necessary we use templates to not impose runtime overhead. In code sections that are not runtime-critical we use conditions and switches over an enum that indicates which generational collection mode we are in (young or global).
>> 
>> We temporarily make all collections in generational mode young collections. Global and old collections will come later.
>
> Bernd Mathiske has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove superflous empty lines.

Looks good to me! Thank you!

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Marked as reviewed by rkennke (Reviewer).

PR: https://git.openjdk.java.net/shenandoah/pull/8