[foreign-memaccess] RFR 8224614: Cleanup MemoryScope and its implementation

[Maurizio Cimadamore]

Hi,
this patch implements the approach described in [1].

I've refactored MemoryScopeImpl into an abstract class 
(AbstractMemoryScopeImpl) and two concrete subclasses:

* GlobalMemoryScopeImpl - for global scopes (roots of scope hierarchies)
* ConfinedMemoryScopeImpl - for 'local' scopes (created using fork)

The former is shared across multiple threads, but there's no need for 
synchronization as there's no mutable state (as global scopes can't be 
closed).

The latter is thread-confined - which means only the thread owner (which 
is established at scope-creation time) can do scope operations such as 
fork/allocate/merge/close.

I've also cleaned up the various characteristics flags in MemoryScope; 
some of those made no longer sense, since we decided against having 
VarHandle for reading/writing addresses directly. I kept the following:

* PINNED - used to mark scopes that cannot be closed; it's a property of 
global scopes and cannot be set w/o super-user powers

* IMMUTABLE - means that the underlying memory cannot be written to

* UNALIGNED - means that we allow memory writes when addresses do not 
conform to the alignment requirements of the layout from which the 
VarHandle was created

* CONFINED - means that memory associated with the scope can be accessed 
only within the owning thread

All characteristics are disabled by default - and it's up to the client 
to set them. There's no 'inheritance' of characteristics either from 
parents to children.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8224614/

Cheers
Maurizio

[1] - 
https://mail.openjdk.java.net/pipermail/panama-dev/2019-May/005494.html