RFR: Generational support for weak roots and references

[William Kemper]

### Summary
The LRB for non-strong references is modified to permit resurrection of objects outside the generation being collected. In other words, resurrection is only blocked for unmarked objects in the generation being collected.

Each `ShenandoahGeneration` has its own reference processor instance. In some cases, a reference from the old generation may end up on the young generation discovered list if the reference points to a young referent (this would happen if the old reference is in the remembered set). However, young references that point to referents in the old generation are _not_ discovered. This has the effect of strongly marking the old generation referent. This also avoids the case of having young references on the old generation discovered list being evacuated/relocated while they wait for old generation reference processing (although we believe this case would be handled correctly by the existing update references code).

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Commit messages:
 - Configure soft reference policy for old generation
 - Merge branch 'shenandoah' into generational-weak-refs
 - Perform coalesce and fill after class unloading
 - Formatting fixes, remove TODO
 - Do not discover referents outside the active generation
 - Move reference processor into generation abstraction
 - Fix indentation, use override keyword rather than redundant virtual
 - Rename method for consistency with existing APIs
 - Improve type safety for affiliation_name API
 - Coalesce and fill after processing old weak roots to preserve existing assertions
 - ... and 6 more: https://git.openjdk.java.net/shenandoah/compare/d775516e...80821d08

Changes: https://git.openjdk.java.net/shenandoah/pull/53/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=53&range=00
  Stats: 143 lines in 21 files changed: 77 ins; 22 del; 44 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/53.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/53/head:pull/53

PR: https://git.openjdk.java.net/shenandoah/pull/53