Firestore: minor tweaks to ThreadSafeMemozier code in response to code review feedback

[dconeybe]

This PR addresses some minor feedback in #14300, the refactor of ThreadSafeMemoizer to fix the apparent "memory leak" detected by Xcode.

The changes are as follows:

1. Rename CountingFunc::Next() to CountingFunc::NextFuncReturnValue().
2. Use const vectors for memoized values; that is, change std::shared_ptr<std::vector<T>> to std::shared_ptr<const std::vector<T>>.
3. Remove an obsolete inline comment about an implementation choice pertaining to the old implementation of ThreadSafeMemoizer.

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