Single-waiter mutex

[itrofimow]

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There are some situations when a resource is accessed concurrently, but concurrency factor is limited by 2, - implications of socket r/w duality come to mind, - and engine::Mutex seems like a go to choice (and it is), however it operates over full-featured WaitList, when in this specific case WaitListLight would do, since there's at most 1 waiter at any given moment.