chore: update CMC for multi-threading

[ajewellamz]

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[ajewellamz]

Question: Should there not be some general statement like: The Cryptographic Materials Cache MUST support Multiple Threads.

I'm not sure. CMC is an abstract class, Maybe we want to allow an implementation that is optimized for single threaded use. The plan is already to have CreateCryptographicMaterialsCache return either a SynchronizedLocalCMC or a StormTrackingCMC. Maybe returning a LocalCMC would be appropriate under some circumstances.

I don't know the right answer.

[texastony]

Hmm... Maybe we should have a more generic requirement. i.e: The MPL MUST include a CMC implementation that supports Multiple Threads.