Statistics collection and thread safety fixes for AZ::IO::DedicatedCache

[dkondrashkin]

More fixes for problems addressed by https://github.com/aws/lumberyard/pull/494

While previous pull request fixed most of the issues the one remained:

• DedicatedCache::DestroyDedicatedCache() call can happen right between DedicatedCache::CollectStatistics() call and any code that makes use of statistics collected (e.g. System.cpp @ 1745 or Device.cpp @ 96). This leads to accessing invalid memory through string views stored in statistics values and, particularly, to crashes on consoles.

The solution proposed (apart form guarding internal data members with locks) is to postpone removal of m_cachedStatNames entries until the very beginning of the next DedicatedCache::CollectStatistics() call.

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[AMZN-alexpete]

Thanks for submitting these additional fixes @dkondrashkin, we're looking over the change and will update this PR with status changes or questions.