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

[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.

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Commit messages:
 - Add reachability fences on all handles
 - Make sure that handles keep a strong reference on their scopes

Changes: https://git.openjdk.java.net/panama-foreign/pull/476/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=476&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264176
  Stats: 64 lines in 4 files changed: 42 ins; 9 del; 13 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