RFR: 8277372: Add getters for BOT and card table members

[Vishal Chand]

Changed the visibility, added getters and refactored the following:

1. Card Table Members
2. BOT members
3. ObjectStartArray block members

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Commit messages:
 - Initial patch

Changes: https://git.openjdk.java.net/jdk/pull/6570/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6570&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277372
  Stats: 199 lines in 31 files changed: 40 ins; 11 del; 148 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6570.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6570/head:pull/6570

PR: https://git.openjdk.java.net/jdk/pull/6570