Improve performance of creating proxy object.

[zaki50]

To support default value feature (#777), #3397 changes the way to create the proxy instance. That introduced about 10-15% unexpected overhead for creating proxy instance.

One of the reason of performance degrade is accessing the thread local. I modified #3397 no to use thread local and it improves the performance about 7-10%.

The idea is

1. Making a state class for each proxy class
2. Move the proxy class to inner class of its state class
3. Create State instance first, and then create proxy instance.

I think proxy classes can obtain object context information from its enclosing instance.

[Zhuinden]

This matters only in case of realm.createObject(), right?

So realmResults.get(i) is not impacted?

[zaki50]

@Zhuinden Unfortunately, it affects all APIs that create proxy object.

If my idea works as expected, I think that almost all overhead will be removed.