Keep track of pending Redis get calls in client side caching to avoid race with invalidations. #3481

[IvarHenckel]

Closes #3481

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[IvarHenckel]

So far didn't add any tests. I felt I was only changing internal parts and there is already tests against the end behavior. Testing the race itself, i.e. somehow mocking the order of threads, is tricky. But let me know if you prefer more tests.

[IvarHenckel]

FYI I have been applying the same fix in my production code, it has been running there for weeks successfully. Would be nice to clean up the code by getting this fix into lettuce (currently have the implementation of CacheAccessor in our code base). And of course, to fix the issue for others.