[foreign-jextract] RFR: MemorySegmentPool + Allocator [v8]

[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:

  Tests better distribute sizes for pseudo random allocation - now much larger buffers gets allocated and used.
  
  I'm bit puzzled here, as it looks like, that allocatioin is very fast, only
  reading is extreme slow.
  
  For comparision when processing single allocation:
  Update only first byte: direct pool access - 33ns, malloc / free: 59ns
  Update very 1024th byte: 12980ns, vs malloc: 8400ns
  
  I think simillar results has been spot during test with StringLen.

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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/4faf2098..cf5c2e02

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=509&range=07
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=509&range=06-07

  Stats: 53 lines in 1 file changed: 23 ins; 10 del; 20 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