Configurable acknowledgement mode for kafka binding

[jfallows]

In kafka binding client, the producer currently defaults to acks -1 (all = in sync replicas).

Mapping bindings that target kafka for produce, such as http-kafka binding, can be enhanced to configure acks in their with clause when following a route.

{
    "bindings":
    {
        "http_kafka_proxy0":
        {
            "type": "http-kafka",
            "kind": "proxy",
            "routes":
            [
                {
                    "when":
                    [
                        {
                            "path": "/items/{id}"
                        }
                    ],
                    "exit": "kafka_cache_client0",
                    "with":
                    {
                        "capability": "produce",
                        "topic": "items-requests",
                        "key": "${params.id}",
                        "acks": "leader_only",
                        "reply-to": "items-responses"
                    }
                }
            ]
        }
    }
}

This will require changes to KafkaMergedBeginEx and KafkaProduceBeginEx to convey the required acks mode, also internally in the kafka cache_server and cache_client to propagate the acks mode across the cache.

As part of implementing this enhancement, the default acks mode should remain as in_sync_replicas.