8357249: Compiler task keeps --system files open

[david-beaumont]

Version of #2374 which rewrites the initialization to clarify the way buffers are used. This class is historically very complex and, in mainline, contains issues around whether files should or should not be memory mapped, and how much of the file exists in the "memoryMap" field (which may not even be a memory mapped buffer).

Now, if non-null, the memoryMap field only contains the entire buffer that's mapped.
Under all other circumstances, the channel exists and is used to read buffer entries piecemeal.

This makes reasoning about who owns the buffer and how its lifecycle is managed much easier and makes it clear that non-runtime (target) images are only ever loaded piecemeal to avoid opening a memory mapped file.

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Progress

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Issue

• JDK-8357249: Compiler task keeps --system files open (Bug - P4) ⚠️ Issue is already resolved. Consider making this a "backport pull request" by setting the PR title to Backport <hash> with the hash of the original commit. See Backports.

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