[foreign-memaccess] RFR: Simplify MemoryScope implementation to use StampedLock

[Maurizio Cimadamore]

I've been playing around a bit and discussed this offline with @JornVernee too. A realization I had some time ago is
that what we're trying to do here is a read/write lock, where multiple acquire can occur at the same time, but where
close of the root should be exclusive. I think all the machinery around `CLOSING` is essentially due to the code trying
to mimic that pattern.

This patch simplifies over the status quo, by using a `StampedLock` instead of managing races manually through a
`CLOSING` state. The tweaked implementation also adds back the atomicity w.r.t. close() vs. dup() which was seeked in a
previous attempt (see https://git.openjdk.java.net/panama-foreign/pull/160). Also, since now there some guarantee that
no acquire can take place while a close is also taking place (and pending acquires will be invalidated - using the
optimistic read logic), then we can just use a single long adder instead of two.

The state is also simplified, since we just need a boolean flag.

I ran the `ParallelSum` benchmark and could not spot any obvious regression compared to the previous code.

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Commit messages:
 - Simplify memory scope
 - * fix exception message check in test

Changes: https://git.openjdk.java.net/panama-foreign/pull/163/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/163/webrev.00
  Stats: 111 lines in 3 files changed: 67 ins; 14 del; 30 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/163.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/163/head:pull/163

PR: https://git.openjdk.java.net/panama-foreign/pull/163