[foreign-jextract] RFR: MemorySegmentPool + Allocator [v7]

[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 two additional commits since the last revision:

 - Better releasing resources on scopes close
   New benchamarks for bulk allocations
 - Minor fixes to SpinLockQueue
   Remove redundant LOCK release (set to 0) in put entry.
   Fixed maxSize comparison
   Removed unused imports
   
   Q: Could we reduce setRelease to just set in few places?
   Q: Should this method be private and moved to Entry.close to prevent accidental adding element from other queue?

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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/ea743bfe..4faf2098

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=509&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=509&range=05-06

  Stats: 334 lines in 3 files changed: 297 ins; 4 del; 33 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