RFR: 8320215: HeapDumper can use DumpWriter buffer during merge

[Chris Plummer]

On Fri, 19 Apr 2024 00:10:12 GMT, Alex Menkov <amenkov at openjdk.org> wrote:

> 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

Looks good.

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PR Review: https://git.openjdk.org/jdk/pull/18850#pullrequestreview-2089482773