[foreign-memaccess+abi] RFR: 8264176: ResourceScope handles should keep parent scope reachable [v2]

[Maurizio Cimadamore]

> While ResourceScope handles are handy to ensure that a scope is not closed too soon, they don't do much in order to help preventing the scope to become unreachable. This means that if a scope supports both deterministic and implicit deallocation, there might be cases where using a try-with-resources with a resource scope handle is not enough (as the scope might become unreachable inside the TWR block).
> 
> The solution is to add reachability fences in the various handle implementations, so that the parent scope is kept reachable.
> 
> I've refactored the implementation a bit, to remove the "closeable" flag - which was generally not used (only default scopes and global scope are non closeable). So, I have created a new class which <: SharedScope, `NonCloseableScope`. Both global scope and default scope are subclasses of that. This class recycles the handles it generates - e.g. only one is created, after which the same handle instance is returned.
> 
> An alternative would have been to make NonCloseableScope a subtype of both MemoryScope and ResourceScope.Handle - but that's difficult since both are AutoCloseable and feature a `close` method with different semantics.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Address renaming as per review comment

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Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/476/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/476/files/bc405e76..29e3267d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=476&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=476&range=00-01

  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/476.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/476/head:pull/476

PR: https://git.openjdk.java.net/panama-foreign/pull/476