add options for exchange durability and sending the payload only

Not all exchanges are created durable, so the `exchange_durable` option allows you to specify this.

Also in some cases, you may only want to transmit the payload data over AMQP, especially since the tag is already the key. With the `payload_only` option, you will lose the original timestamp but have a simpler data structure.

Default behavior is the same for both options.