RFR: 8276670: G1: Move and rename G1CardSetFreePool and related classes

[Hamlin Li]

This is to simply move and rename G1CardSetFreePool and related classes, as G1CardSetFreePool and related classes are going to be reused outside of the remembered set, they should be factored out and renamed.
Rename from G1CardSetXxx -> G1BufferListXxx.

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Commit messages:
 - Fix typos
 - Fix gtest tests
 - Fix missing in test
 - Fix missing files
 - move g1CardSetFreeMemoryTask.*
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6289/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6289&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276670
  Stats: 1228 lines in 20 files changed: 641 ins; 554 del; 33 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6289.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6289/head:pull/6289

PR: https://git.openjdk.java.net/jdk/pull/6289