Use GC to close RealmReferences

[clementetb]

Currently, we use the VersionTracker to close realms whose RealmReferences that have been claimed by the GC.

This has two effects:
first versions will not be reclaimed until there is some version advancement;
second, closing realms would be done in the notifier/writer threads, instead of using the GC collector.

We should explore moving close and let the GC thread close and reclaim realms.

[cmelchior]

Hmm, not sure I agree with this. Having a deterministic lifecycle for these versions is pretty nice and makes it easier to reason about what is happening. But we can discuss it.