StringDedupTable should use Access API

The StringDedupTable class should be using the Access API for all accesses to the StringDedupTableEnty::_obj member. Presently it only uses Access in the lookup() function (and incorrectly there, until JDK-8205559 is fixed).

There is also a lingering reference to the non-existent G1StringDedupTableEntry class that should be renamed. (It doesn't cause a compile error because it is used as an ignored macro argument.)