[foreign-memaccess] RFR: JDK-8252757: Add support for shared segments (followup)

[Maurizio Cimadamore]

I realized there's a small issue in the API code for shared segment support: the liveness test is always performed
inside MemoryScope (e.g. on share(), confineTo() and close()). While this is ok, there is an usability issue, in that
the `checkValidState` method inside MemoryScope throws raw scoped exceptions (as this routine is really meant only to
be used during memory access).

A better solution is to move the liveness check outside the scope implementation for these operations, and do the check
in AbstractMemorySegment, which correctly wraps the exception.

I've added a comprehensive test which checks that no raw exceptions are leaked from the segment API.

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Commit messages:
 - Fix raw access exception leaking from segment API

Changes: https://git.openjdk.java.net/panama-foreign/pull/318/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=318&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252757
  Stats: 26 lines in 3 files changed: 19 ins; 7 del; 0 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/318.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/318/head:pull/318

PR: https://git.openjdk.java.net/panama-foreign/pull/318