[foreign-memaccess+abi] RFR: MemorySegmentPool + Allocator [v12]

[Radoslaw Smogura]

> (Preview)
> 
> The MemorySegmentPool is a pool maintaining memory segments, optionally can expose allocator which can be bound to other scope, and which will return allocated segments back to pool.
> 
> However the best results has been achieved by using getSegmentEntry & putSegmentEntry methods.
> 
> The pool is intended to be used by long running applications (i.e. like global shared pool), where fast allocation and de-allocation of segments is critical (was designed during implementation of I/O subsystem with Panama, as a pool for temporary buffers between system I/O methods and Java byte arrays from InputStreams).
> 
> The pool uses hand-made SpinLockQueue as the Deque from JDK offers too much functionality and overhead.

Radoslaw Smogura has updated the pull request incrementally with one additional commit since the last revision:

  Drop internal API.
  Use fields access instead of VarHandler in poll & put entry (small performance gain,
  hope fences will work normally)
  
  Replace scope close handler with ResourceCleanup to get additional performance
  
  Dropped registering cleaners per allocated item, and moved freeing entries to cleaner (thanks Maurizio)

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Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/509/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/509/files/32e307b6..fc603b9b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=509&range=11
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=509&range=10-11

  Stats: 85 lines in 4 files changed: 12 ins; 52 del; 21 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/509.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/509/head:pull/509

PR: https://git.openjdk.java.net/panama-foreign/pull/509