First demo

[haaawk]

This is a top-level issue for the first iteration that ends with a working demo.

The goal is to create a kafka_producer class that has the following member functions:

#include <seastar/core/sstring.hh>

class kafka_producer {
public:
  using bytes = basic_sstring<int8_t, uint32_t, 31, false>;
  using bytes_view = std::basic_string_view<int8_t>;
  future<> init(kafka_producer::properties& p);
  future<> close();
  future<bool> send(const sstring& topic_name, bytes key, bytes value);
  future<bool> send(const sstring& topic_name, bytes_view key, bytes_view value);
}

Usage of this class should be demonstrated with a standalone seastar command-line program that reads data from the standard input line by line and sends it to Kafka. It should operate in a loop similar to:

kafka_producer p;
p.init(/* ... */);
while (true) {
  auto key = read_line();
  auto value = read_line();
  p.send(topic_name, key, value);
}

It should be able to start this program with:
kafka-demo <kafka_node_ip> <topic_name>

It should be possible to verify that data has ended up on Kafka with:
bin/kafka-console-consumer.sh --bootstrap-server <kafka node ip>:9092 --topic <topic_name> --from-beginning

Tasks:
#26 Write standalone command-line demo client for kafka_producer
#27 Implement kafka_producer::init function
#28 Implement kafka_producer::send function