Fix race in cache eviction

[majocha]

I spotted a bug in the recently merged general purpose cache.

Let's fix it before folks start complaining.

[github-actions]

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[majocha]

The bugfix. We obviously don't want to lock the ConcurrentDictionary for eviction queue updates.
This makes a race possible where an Update is posted just as the item was being evicted.
In effect Node.List was null, and the mailbox processor silently crashes.
This would happen during intensive cache use with evictions going on, for example when switching branches or solutions in the IDE.

The symptom of this would be excessive allocations. Or lack of eviction if you take a look at the metrics.

This is tested manually in the IDE and confirmed that no longer reproduces.