Support for Drain mode

[wallyqs]

Adds new drain API which a client can use to close a connection gracefully, allowing subscribers to process pending messages that have already been delivered by the server, done by announcing first to the server that no longer interested in receiving messages in all the subscriptions.

Unlike the Go client, it is only possible to drain the full connection since current implementation relies on a single buffer for all the subscriptions.

NATS.start(drain_timeout: 1) do |nc|
  NATS.subscribe('foo', queue: "workers") do |msg, reply, sub|
    nc.publish(reply, "ACK:#{msg}")
  end

  NATS.subscribe('bar', queue: "workers") do |msg, reply, sub|
    nc.publish(reply, "ACK:#{msg}")
  end

  NATS.subscribe('quux', queue: "workers") do |msg, reply, sub|
    nc.publish(reply, "ACK:#{msg}")
  end

  EM.add_timer(2) do
    next if NATS.draining?

    # Drain gracefully closes the connection.
    NATS.drain do
      puts "Done draining. Connection is closed."
    end
  end
end