Improve stability on AppDomain reloads.

[kring]

Fixes #525
(at least, it should be much better!)

This PR makes two changes that should significantly decrease the chances of a crash when Unity reloads AppDomains:

• Uses the Impl class reference counting added in Reference counting and accidental copy prevention for Impl classes #503 to ensure that the CesiumIonSessionImpl instance will not be destroyed while the network requests it starts are in progress.
• Catches the exception in more cases that is returned by GCHandle.FromIntPtr when the AppDomain has been reloaded and the IntPtr corresponds to an object from the old domain. Exceptions are disastrous and often create instability because of Add support for exceptions to Reinterop #18.

Unity reloads AppDomains a lot, so anything we can do to more it work more reliably is likely to be a big win for users. Unfortunately, this probably isn't a 100% solution. We really need #18, for one thing. For another, it's not clear to me (even after looking at the Mono source code) whether the "object is from a previous AppDomain" detection is 100% reliable. It might be best effort, and sometimes just give you the "wrong" object instead of throwing an exception.

• https://github.com/Unity-Technologies/mono/blob/2021.3.13f1/mcs/class/corlib/System.Runtime.InteropServices/GCHandle.cs#L134
• https://github.com/Unity-Technologies/mono/blob/2021.3.13f1/mono/metadata/gc.c#L822
• https://github.com/Unity-Technologies/mono/blob/2021.3.13f1/mono/metadata/null-gc-handles.c#L356

If that's the case, we might ultimately need to either: 1) implement a way to reliably stop and unload all of the Native world when the AppDomain unloads, or 2) extend our native ObjectHandle to record the AppDomain.Id and verify it's still correct before dereferencing. Option (2) is probably more realistic.

This PR also updates cesium-native to the current head of main.

[kring]

I'm merging this for tomorrow's release.