completely remove guava related classes and use caffeine backed cache#207
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Replace computeIfAbsent-based load coalescing with an external ConcurrentHashMap<K, CompletableFuture<V>> in-flight map. Previous approach use user-supplied loader inside the ConcurrentHashMap mapping function, which holds a bin-level lock during computation. This has two problems for a general-purpose cache API: - A slow loader blocks all map operations for keys in the same bin - A loader that calls back into the same cache can deadlock on ConcurrentHashMap The new approach: 1. Atomically putIfAbsent a CompletableFuture into the in-flight map 2. The winning thread runs the loader with no locks held 3. Losing threads join() the existing future 4. CompletionException is unwrapped so all callers see the same exception type regardless of whether they were the loader or waiter 5. remove(key, fresh) in finally ensures only the owning future is cleaned up, preventing accidental clobber of a concurrent reload Coalescing semantics are preserved: concurrent cold misses for the same key still trigger exactly one loader invocation.
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