RFR 9: 8138696 : java.lang.ref.Cleaner - an easy to use alternative to finalization

[Roger Riggs]

Please review a few improvements to the Cleaner nested classes and 
implementation.
  - The extensible reference classes have shorter names that focus on 
the cleanup behavior,
     PhantomCleanup, WeakCleanup, SoftCleanup and to update the javadoc
  - Added documentation for the Reference.get method; its behavior was 
not clear
  - Corrected a race condition that would prevent the cleaning thread 
from terminating
  - cleanup of the link list management

Webrev:
http://cr.openjdk.java.net/~rriggs/webrev-cleaner-extensible-8138696/

javadoc:
    http://cr.openjdk.java.net/~rriggs/cleaner-doc2/

Please review and comment, Roger