RFR: 8334342: Add MergeStore JMH benchmarks [v2]

[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 four additional commits since the last revision:

 - use VarHandler CHAR_L & CHAR_B
 - copyright
 - bug fix for putIntBU
 - add cases for `getChar` & `putChar`

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Changes:
  - all: https://git.openjdk.org/jdk/pull/19734/files
  - new: https://git.openjdk.org/jdk/pull/19734/files/72a6256a..65bb597d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19734&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19734&range=00-01

  Stats: 322 lines in 1 file changed: 276 ins; 14 del; 32 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