RFR: 8314021: HeapDump: Optimize segmented heap file merging phase [v2]

[Yi Yang]

> This patch reduce ~16%(24s->20s) pahse 2 merge time during dumping 32g heap with 96threads and fixes a memory leak of compressor
> 
> You might argue why this is Linux-only optimization, because sendfile requires at least socket fd in other platforms([aix sendfile](https://www.ibm.com/docs/en/aix/7.1?topic=s-send-file-subroutine) [maxos sendfile](https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/sendfile.2.html) [win32 TransmitFile](https://learn.microsoft.com/en-us/windows/win32/api/mswsock/nf-mswsock-transmitfile)), while [only Linux](https://man7.org/linux/man-pages/man2/sendfile.2.html) supports both two file descriptors.

Yi Yang has updated the pull request incrementally with one additional commit since the last revision:

  one merge_file for all

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/15245/files
  - new: https://git.openjdk.org/jdk/pull/15245/files/ba381aaf..0eb02c14

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15245&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15245&range=00-01

  Stats: 16 lines in 1 file changed: 4 ins; 9 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/15245.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15245/head:pull/15245

PR: https://git.openjdk.org/jdk/pull/15245