RFR: 8277542: G1: Move G1CardSetFreePool and related classes to separate files

[Hamlin Li]

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

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6499/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6499&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277542
  Stats: 582 lines in 4 files changed: 326 ins; 256 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6499.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6499/head:pull/6499

PR: https://git.openjdk.java.net/jdk/pull/6499