RFR: 8333457: GenShen: Move remembered set into new generational code [v3]

[William Kemper]

> The remembered set is moved into the old generation and the age census is moved into the generational mode heap.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Improve variable names, comments and add assertions

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/443/files
  - new: https://git.openjdk.org/shenandoah/pull/443/files/954be8eb..2b1d49ca

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=443&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=443&range=01-02

  Stats: 25 lines in 3 files changed: 5 ins; 0 del; 20 mod
  Patch: https://git.openjdk.org/shenandoah/pull/443.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/443/head:pull/443

PR: https://git.openjdk.org/shenandoah/pull/443