Review Request JDK-8201793: (ref) Reference object should not support cloning

[mandy chung]

The semantics of cloning a Reference object is not clearly defined. In 
addition, it is questionable whether it can be meaningfully supported in 
particular with concurrent reference processing.

The reachability state of a Reference object may change during GC 
reference processing. The referent may have been cleared when it reaches 
its reachability state. In addition, it may be enqueued or pending for 
enqueuing. Cloning a Reference object with a referent that is 
unreachable but not yet cleared might mean to resurrect the referent. A 
cloned enqueued Reference object will never be enqueued.  A Reference 
object cannot be meaningfully cloned.

We propose Reference::clone to throw CloneNotSupportedException.  To 
clone a Reference object, it should create a new instance via its 
constructor.

CSR: https://bugs.openjdk.java.net/browse/JDK-8202260

Webrev:
http://cr.openjdk.java.net/~mchung/jdk11/webrevs/8201793/webrev.00/

Mandy