8026344: j.u.c.a *Adder and *Accumulator extend a package private class that is Serializable

[Alan Bateman]

j.u.c.atomic.{Double,Long}.{Accumulator,Adder} extend a package-private 
class Striped64 that does striping on 64-bit values. This super type is 
Serializable by way of extending Number. Striped64 doesn't contribute 
any serial fields but its name does leak into the serial stream because 
there is a descriptor for the super type when it is also serializable. 
Those that write conformance tests have noticed this.

In practical terms this is a bit of a non-issue but the reference to 
Striped64 can be removed from the serial stream by changing these four 
to use a serialization proxy. Doug has gone ahead and changed the 
classes in jsr166 CVS and we need to bring these changes into jdk8/tl by 
the ZBB date.

The webrev with the proposed changes is here:

http://cr.openjdk.java.net/~alanb/8026344/webrev/

A minor difference with what is currently in the CVS is that I've added 
a link in the writeReplace javadoc to the proxy class. I've also added a 
simple test as I didn't find any tests in the jdk repository that 
exercise serialization of these classes.

-Alan.