[runtime] Change MonoDomain::refobject_hash to use MonoConcGHashTable.

[runtime] Change MonoDomain::refobject_hash to use MonoConcGHashTable.

This change massively improves reflection query scalability.

Given this benchmark and my 4 cores laptop:

```
const int T_C = 4;
var list = new List<Thread> ();
bool done = false;
int count = 0;
for (int i = 0; i < T_C; ++i) {
	var t = new Thread(() => {
		while (!done) {
			typeof (Driver)
				.GetMethods (
					BindingFlags.Public | BindingFlags.NonPublic |
					BindingFlags.Instance | BindingFlags.Static);
			Interlocked.Increment (ref count);
		}
	});
	t.Start ();
	list.Add (t);
}

Thread.Sleep (5000);
done = true;
foreach (var t in list)
	t.Join ();
Console.WriteLine ("Got {0} lists done", count);
```

This change makes us go from 166.725 queries/s to 5.140.821 queries/s. Or 30x faster.