Improve concurrency beyond a single lock

You are given a simple multi-threaded component that protects all shared state with a **single mutual-exclusion lock (mutex)**. ### Scenario A service has many worker threads calling `get(key)` and `put(key, value)` on a shared in-memory key–value store (e.g., a hash map). The current implementation uses **one global mutex** around the entire `get/put` logic to ensure correctness. ### Tasks 1. Explain why the single global lock is correct but can hurt performance under contention. 2. Propose concrete ways to **improve concurrency / throughput** while keeping correctness. Discuss trade-offs. 3. Call out common pitfalls (deadlocks, race conditions, starvation) and how you would test/validate the improved design.