RFR: 8334342: Add MergeStore JMH benchmarks [v4]

[Shaojin Wen]

> [8318446](https://github.com/openjdk/jdk/pull/16245)  brings MergeStore. We need a JMH Benchmark to evaluate the performance of various batch operations and the effect of MergeStore.

Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:

  bug fix for `putChars4C`

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Changes:
  - all: https://git.openjdk.org/jdk/pull/19734/files
  - new: https://git.openjdk.org/jdk/pull/19734/files/1ed9b22e..1140bd81

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19734&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19734&range=02-03

  Stats: 8 lines in 1 file changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/19734.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19734/head:pull/19734

PR: https://git.openjdk.org/jdk/pull/19734