Protect against double open channel

[carlhoerberg]

Looks like there's no protection from the client opening the same channel twice:

lavinmq/src/lavinmq/client/client.cr

Lines 314 to 318 in f132fb9

	private def open_channel(frame)
	@channels[frame.channel] = Client::Channel.new(self, frame.channel)
	@vhost.event_tick(EventType::ChannelCreated)
	send AMQP::Frame::Channel::OpenOk.new(frame.channel)
	end

[carlhoerberg]

Not sure if it's allowed to double open a channel, but at least the channel object in lavinmq shouldn't be overwritten.