Review request for JDK-8067215: Disable dual fields when not using optimistic types

[Hannes Wallnoefer]

Please review JDK-8067215: Disable dual fields when not using optimistic 
types:

http://cr.openjdk.java.net/~hannesw/8067215/

This makes single or dual field representation dependent on optimistic 
types setting (can be overridden with system properties).

Since both dual and single field objects can coexist in the same VM and 
class loader and both have to be distinguishable at runtime (e.g. for 
caching getter and setter methodhandles in AccessorProperty) I had to 
use separate class names for both. I introduced 
jdk.nashorn.internal.scripts.JD for dual fields while using the existing 
JO name scheme for single field representation. I also added a new 
property flag as a property with dual fields must not be equal to the 
same property with object only fields.

On the other hand, nasgen-generated properties always use their native 
type, i.e. primitive accessors if their type is primitive and object 
accessors otherwise. This is because nasgen-generated property maps are 
static and have to work in both modes, and using them in non-optimistic 
mode requires boxing/unboxing anyway.

I also did some cleanup in related areas such as not creating a 
primitive spill in single field mode and removing 
ScriptObject.spillLength field which was not really needed. I see an 
improvement in non-optimistic performance with octane. As expected, 
optimistic performance does not seem to be affected.

Thanks,
Hannes