RFR: Differentiate young generation marking. [v3]

[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 incrementally with one additional commit since the last revision:

  Remove superflous empty lines.

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Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/8/files
  - new: https://git.openjdk.java.net/shenandoah/pull/8/files/d6393839..ca95ebd4

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=8&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=8&range=01-02

  Stats: 4 lines in 1 file changed: 0 ins; 4 del; 0 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