[foreign-memaccess] RFR: Preserve memory scope for buffer segments

[Maurizio Cimadamore]

Not 100% sure about this, but wanted to share this PR for evaluation. Basically, I've realized that we are also not
preserving the memory scope of the original segment when we do a segment -> buffer -> segment transition.

This means that, e.g. code like this:

MemorySegment segment = MemorySegment.allocateNative(16);
segment = MemorySegment.ofByteBuffer(segment.asByteBuffer());
segment.close(); // no cleanup of native memory

will not really 'close' the original segment. Similarly, if the original segment is being worked upon by some other
thread (through a spliterator) it is possible for the morphed segment to lose this info, and to allow a close - if the
scope is created afresh (as is now).

This patch fixes this, but I'm torn as to whether it makes things better; preserving access modes and thread ownership
is mostly about not allowing clients to use the byte buffer escape hatch as a way to defy the segment restrictions. I
don't think preserving scope falls into the same bucket - e.g. this is a separate question as to whether a segment S2
obtained through S1 -> BB -> S2 is just a *view* of S1 (with same temporal bounds), or is just an independent segment,
with new temporal bounds. I think both answers are legitimate, and it's mostly a question of making up our mind.

My take is that inferring scope like this can be seen a bit too magic - and you are never sure of what happens when you
do a MemorySegment::close, so perhaps what we have w/o this patch is preferrable. What do you think?

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

Commit messages:
 - Preserve memory scope for buffer segments

Changes: https://git.openjdk.java.net/panama-foreign/pull/113/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/113/webrev.00
  Stats: 32 lines in 2 files changed: 25 ins; 2 del; 5 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/113.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/113/head:pull/113

PR: https://git.openjdk.java.net/panama-foreign/pull/113