RFR (S): 8192003: Refactor weak references in StringTable to use the Access API

[Erik Österlund]

Hi,

The StringTable contains weak references to oops. Today the weak 
semantics is managed using explicit calls to G1 SATB enqueue barriers.

Now that the Access API is available, it should be used instead as it is 
more modular.

This change fixes that by making these oops ON_PHANTOM_OOP_REF with a 
corresponding AS_NO_KEEPALIVE accessor when we do not want to keep it 
alive, much like my previous changes to other weak tables.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8192003/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8192003

Thanks,
/Erik