KIP-261 — Add Single Value Fetch in Window Stores

Accepted Kafka 1.1 Streams

Adds a `fetch(K key, long time)` single-point lookup to the `WindowStore` interface that retrieves the value for an exact window start time, in addition to the existing range query APIs. Without this, fetching the value for a known window required a `fetch(key, from, to)` range query even when the exact window start timestamp is known, unnecessarily scanning multiple windows.

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