KIP-428 — Add in-memory window store

Accepted Kafka 2.3 Streams

Adds an in-memory implementation of the windowed state store for Kafka Streams, available via Stores.inMemoryWindowStore(). Only a persistent (RocksDB-backed) windowed store existed, but some use cases — short retention windows, latency-sensitive stateful ops — benefit from keeping all state in heap memory without the serialization overhead of RocksDB.

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