Add a new configuration value to run multiple consumers in separate threads.

[Adithya-copart]

This is a draft PR to try multiple consumers using threads.

1. Support multiple consumers using threads when threaded option is passed.
2. Shutdown all the consumers in different threads.

This is tested using rdkafka-ruby:master branch that supports JRuby.

## test_multiple.rb

Racecar.configure do |config|
  # Each config variable can be set using a writer attribute.
  config.brokers = ['localhost:9092']
end

class TestConsumer1 < Racecar::Consumer
  subscribes_to "topic1"
  group_id = 'test1'

  def process(message)
    p "#{self.class} #{message.value} #{message.offset}"
  end
end

class TestConsumer2 < Racecar::Consumer
  subscribes_to "topic2"
  group_id = 'test2'

  def process(message)
    p "#{self.class} #{message.value} #{message.offset}"
 end
end

Command:

bundle exec racecar --require ./test_multiple.rb --threaded TestConsumer1 TestConsumer2

Result:

$ bundle exec racecar --require ./test_multiple.rb --threaded TestConsumer1 TestConsumer2
=> Starting Racecar consumer TestConsumer1, TestConsumer2...
=> Wrooooom!
=> Ctrl-C to shutdown consumer
Starting consumer TestConsumer1
The signal QUIT is in use by the JVM and will not work correctly on this platform
The signal USR1 is in use by the JVM and will not work correctly on this platform
Starting consumer TestConsumer2
W, [2020-03-10T16:35:13.388905 #372]  WARN -- : ActiveSupport::Notifications not available, instrumentation is disabled
W, [2020-03-10T16:35:13.389168 #372]  WARN -- : ActiveSupport::Notifications not available, instrumentation is disabled
^CI, [2020-03-10T16:35:15.001110 #372]  INFO -- : Gracefully shutting down TestConsumer2
I, [2020-03-10T16:35:15.001110 #372]  INFO -- : Gracefully shutting down TestConsumer1
E, [2020-03-10T16:35:15.006739 #372] ERROR -- : rdkafka: [thrd:GroupCoordinator]: 1/1 brokers are down
I, [2020-03-10T16:35:15.006740 #372]  INFO -- : Graceful shutdown of TestConsumer1 completed
I, [2020-03-10T16:35:15.007616 #372]  INFO -- : Graceful shutdown of TestConsumer2 completed

[dasch]

@Adithya-copart would you be interested in restarting this work?

[dasch]

Specifically: scoping out support for running different consumers in a single process, but instead having a single consumer process its assigned topic partitions concurrently.

[adi-pen]

Sorry for the super late reply but I'm no longer using the handle @Adithya-copart.
I'm not using JRuby or racecar anymore and will not be able to do this.