Rework handle ownership

[nirinchev]

Description

This is a major change that reshuffles the handle hierarchy quite a bit. It's very much a WIP, but would appreciate any early feedback.

• Splits native handles into three distinct groups
  • Standalone types (StandaloneHandle) - e.g. User, App, ThreadSafeReference - those are types that exist on their own and have no ownership semantics with other types. Those are also typically not disposable.
  • Realm types (RealmHandle) - e.g. Object, RealmList, Results, etc. - those types are owned by a Realm instance and their lifetime is bound to that instance.
  • Realm (SharedRealmHandle) - this is a standalone type, but it takes care of unbinding its child handles at opportune times.
    • UnownedRealmHandle - this is a subtype of SharedRealmHandle that doesn't delete its native pointer because it wasn't the SDK that created it. Additionally, it keeps a weak list of all its children and closes them as soon as it gets closed. This is used in migrations, where we don't want to have objects outliving the Realm instance as this may keep the native instance alive and cause all sorts of issues.

Discussion items:

• Currently, objects are bound to an instance, not native Realm. This means that disposing an individual instance will invalidate all objects that were created for it, even if the native Realm is still open:

  var realm1 = Realm.GetInstance();
  var realm2 = Realm.GetInstance();
  
  var obj = realm1.All<Foo>().First();
  realm1.Dispose();
  
  // obj is invalid here, even though the native Realm is still open

  I believe this is the intuitive and intended behavior, but its a breaking change compared to before (though the old behavior was accidental and not something we've ever officially supported).
• Not a fan that we now need to call EnsureValid() on every handle method. It's easy to forget and if we do, attempting to call the method will throw a vague ObjectDisposedException. I tried a bunch of clever approaches, but wasn't happy with any of them. Open for smart ideas.
• Right now, we don't ever prune the unowned handle list, which means that if there's a migration that instantiates millions of objects, we may run into trouble.

TODO

• Changelog entry
• Tests (if applicable)

[coveralls]

Pull Request Test Coverage Report for Build 1776006405

• 227 of 239 (94.98%) changed or added relevant lines in 22 files are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.06%) to 82.913%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
Realm/Realm/Handles/ObjectHandle.cs	24	25	96.0%
Realm/Realm/Handles/StandaloneHandle.cs	8	11	72.73%
Realm/Realm/Handles/SetHandle.cs	19	23	82.61%
Realm/Realm/Handles/SharedRealmHandle.cs	35	39	89.74%

Files with Coverage Reduction	New Missed Lines	%
Realm/Realm/Exceptions/RealmException.cs	1	41.28%
Realm/Realm/Sync/FlexibleSync/SubscriptionSet.cs	4	94.44%

Totals
Change from base Build 1775866357:	0.06%
Covered Lines:	5519
Relevant Lines:	6542

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💛 - Coveralls

[papafe]

I didn't know about this part of the code. Is it a small subset duplicating what the c# driver does?

[nirinchev]

It's nowhere near as comprehensive as the driver - the server exposes a REST-like API to query MongoDb and we're wrapping that here. But we're not querying MongoDb directly.

[papafe]

We don't check for the root here?