RFR: 8265057: G1: Investigate removal of maintenance of two BOT thresholds

[Ivan Walulya]

Hi all,

Please review this change to remove `_next_offset_index` and maintain only one threshold `_next_offset_threshold` a direct pointer to the BOT entry in the BOT. We derive the next_offset_index from from _next_offset_threshold where required. Performance tests do not show any regression as a result of this change. 

Testing: Tier 1-5

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Commit messages:
 - change asserts
 - Merge branch 'master' into JDK-8265057_BOT_thresholds
 - Merge branch 'master' into JDK-8265057_BOT_thresholds
 - Merge branch 'master' into JDK-8265057_BOT_thresholds
 - remove _next_offset_index

Changes: https://git.openjdk.java.net/jdk/pull/4972/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4972&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265057
  Stats: 30 lines in 3 files changed: 4 ins; 7 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4972.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4972/head:pull/4972

PR: https://git.openjdk.java.net/jdk/pull/4972