INTEXT-104: Kafka: `order` for o-channel-adapter

JIRA: https://jira.spring.io/browse/INTEXT-104

`order` attribute now honored if kafka outbound adapter is connected to a subscribable channel
unit tests and samples are updated

Polishing: use `<xsd:attributeGroup ref="integration:channelAdapterAttributes"/>` for adapter tags to cover `SmartLifecycle` options.