Add support for gracefully stopping a consumer that uses Consumer#each

[ukdave]

Currently if the Consumer#each method is called it will loop indefinitely with no way of gracefully stopping it. Calling Consumer#close closes and destroys the kafka handle, but then causes the app to crash an "Invalid memory access" inside the each loop the next time it calls poll.

This PR adds an internal flag to keep track of whether the each loop is running, and adds a Consumer#stop method to stop it. This can be called from, for example, a signal handler to gracefully stop the consumer.

This PR also adds some extra safety by raising an error if an attempt is made to use the consumer after it has been closed instead of crashing with an "Invalid memory access" error.

require "crafka"

consumer = Kafka::Consumer.new({
  "bootstrap.servers"     => "localhost:9092",
  "client.id"             => "test",
  "group.id"              => "test",
  "broker.address.family" => "v4",
})

Signal::INT.trap do
  Log.info { "Received SIGINT" }
  consumer.stop
end

Log.info { "Subscribing to topic..." }
consumer.subscribe("test-topic")

Log.info { "Starting consumer loop..." }
consumer.each do |kafka_event|
  payload = String.new(kafka_event.payload)
  Log.info { "Consumed event: #{payload}" }
end
Log.info { "Exited consumer loop" }

Log.info { "Closing consumer..." }
consumer.close

Log.info { "Bye" }