Frozen Realm doesn't reference count correctly

[alecbarber]

Creating new instances of an already-cached version doesn't increment the reference count, which causes closing frozen Realm instances to be non-deterministic. The following code works as expected:

val realm = Realm.getDefaultInstance()

// Freeze the Realm
val realm1 = realm.freeze()
// Write to the live Realm, and freeze again
realm.executeTransaction { ... }
val realm2 = realm.freeze()

// Close the first frozen instance
realm1.close()

// Make a query on the second frozen instance
realm2.where<...>().findAll().doSomething()

// Clean up
realm2.close()
realm.close()

However, this code throws an IllegalStateException if the write operation doesn't happen between the two freeze operations:

val realm = Realm.getDefaultInstance()

// Freeze the Realm
val realm1 = realm.freeze()
// Freeze a second time first, then write to the live Realm
val realm2 = realm.freeze()
realm.executeTransaction { ... }

// Close the first frozen instance.
realm1.close()

// Make a query on the second frozen instance
// Throws IllegalStateException, saying that realm2 is closed.
realm2.where<...>().findAll().doSomething()

// Clean up
realm2.close()
realm.close()

In a use-case where frozen RealmObjects/RealmResults are being temporarily created and there are concurrent write transactions (which is for example very easy to achieve with Kotlin Coroutines), cleaning up frozen Realms rapidly becomes non-deterministic.

This PR attempts to fix the issue by making the reference-counting behaviour of frozen Realms match that of live Realms – that is, every Realm instance created with Realm.freeze() is active (and using resources) until close() is called on that instance.

[bmunkholm]

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