RFR: 8282072: G1: Rename CardSetPtr to CardSetContainerPtr

[Ivan Walulya]

Hi all,
Please review this change to rename CardSetPtr to ContainerPtr since it is a pointer to the containers. 

Testing: tier1

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Commit messages:
 - refactor

Changes: https://git.openjdk.java.net/jdk/pull/7816/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7816&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282072
  Stats: 311 lines in 8 files changed: 6 ins; 13 del; 292 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7816.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7816/head:pull/7816

PR: https://git.openjdk.java.net/jdk/pull/7816