RFR: 8320215: HeapDumper can use DumpWriter buffer during merge

[Alex Menkov]

The fix updates HeapMerger to use writer buffer (no need to copy memory, also writer buffer is 1MB instead of 4KB).
Additionally fixed small issue in FileWriter (looks like `ssize_t` instead of `size_t` is a typo, the argument should be unsigned)

Testing: all HeapDump-related tests on Oracle supported platforms

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/18850/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18850&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320215
  Stats: 22 lines in 3 files changed: 9 ins; 4 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/18850.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18850/head:pull/18850

PR: https://git.openjdk.org/jdk/pull/18850