Make snapshot source thread-safe.

[ewoutkramer]

This question was posed on Zulip:

https://chat.fhir.org/#narrow/stream/179171-dotnet/topic/Encountered.20an.20error.20while.20loading.20schema

When the Validate method gets hit under heavy load, it seems that we encounter the following error:

Encountered an error while loading schema 'http://hl7.org/fhir/StructureDefinition/Bundle': Invalid snapshot generator state (OnStartRecursion).
Cannot start a new snapshot generation. The previous operation has not finished.

I assume this is because there are multiple concurrent requests.

This sounds like the recursion-detection state in the snapgen is getting confused. This might be a threading issue indeed.

[mmsmits]

This is due to SnapshotSource not being thread-safe. This can be solves by running multiple validators, but that results in another performance overload.

We could think about making our resolvers thread-safe, because this comes back every time.