RFR: 8276670: G1: Rename G1CardSetFreePool and related classes

[Hamlin Li]

As G1CardSetFreePool and related classes are going to be reused outside of the remembered set, they should be renamed.

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Commit messages:
 - Rename log messages
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6514/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6514&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276670
  Stats: 730 lines in 19 files changed: 332 ins; 316 del; 82 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6514.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6514/head:pull/6514

PR: https://git.openjdk.java.net/jdk/pull/6514