RFR: 8265057: G1: Investigate removal of maintenance of two BOT thresholds [v2]

[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

Ivan Walulya has updated the pull request incrementally with one additional commit since the last revision:

  sreviewer uggestiedto reemove comment

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4972/files
  - new: https://git.openjdk.java.net/jdk/pull/4972/files/4d412780..22afbff9

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4972&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4972&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 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