DSL improvements for broker-backed pub/sub channels.

[garyrussell]

Core (in-memory) pub/sub channels can be configured with in-line subscriptions:

.publishSubscribeChannel(c -> c
		.subscribe(sf -> sf.handle(
				kafkaMessageHandler(producerFactory(), TEST_TOPIC1)
						.timestampExpression("T(Long).valueOf('1487694048633')"),
				e -> e.id("kafkaProducer1")))
		.subscribe(sf -> sf.handle(
				kafkaMessageHandler(producerFactory(), TEST_TOPIC2)
						.timestamp(m -> 1487694048644L),
				e -> e.id("kafkaProducer2")))
)

Message broker-backed pub/sub channels have the same semantics as in-memory pub/sub in an application generally (distribution to multiple subscribers) but there is no support for in-line subscriptions in the DSL.

It may be difficult/impossible to support but we should at least explore the possibility.