Make in-memory key-value store skip serde

SAMZA-256 added an in-memory implementation of the samza-kv store. Due to the layering in Samza's KV-store APIs, the in-memory store still holds raw bytes, and the Serde is used to transform objects back into POJOs. On the read-side, it is unnecessary to deserialize the bytes back into an object. The in-memory KV store should just hold the raw object.

Semantically, this does change the behavior of the KV-store a bit, when using in-memory stores. If an object is mutated after it's been written to an in-memory store, and then store.get is called, the mutated object will be returned. This is not the case with regular (LevelDB/RocksDB) KV-stores.

Writes will still require serializing the object if a changelog is attached. If a changelog is not attached, then I'd argue that the in-memory KV store should not be used at all, and a simple HashMap should be used instead.

When updating the code, we should be mindful to keep the API as clean as possible, while shifting the layers around.