RFR: Differentiate young generation marking. [v2]

[Bernd Mathiske]

> 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 with a new target base due to a merge or a rebase. The pull request now contains one commit:

  Differentiate young generation marking.

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Changes: https://git.openjdk.java.net/shenandoah/pull/8/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=8&range=01
  Stats: 371 lines in 17 files changed: 216 ins; 33 del; 122 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/8.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/8/head:pull/8

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