Refine `ReactiveRedisOperations#listenTo` and `listenTo…Later` Javadoc mentioning resource usage

[vpavic]

Channel multiplexing reference

Even though sections on Message Listener Containers (both imperative and reactive ones) describe resource sharing between multiple listeners, the term multiplexing isn't explicitly called out anywhere and I believe that would be useful thing to have in the docs. Note that class javadocs of both RedisMessageListenerContainer and ReactiveRedisMessageListenerContainer do mention the concept of channel multiplexing.

Warn about pitfalls of ReactiveRedisOperations#listenTo (and other #listenTo variants)

The ReactiveRedisOperations#listenTo API is tempting to use (in comparison with manually setting up ReactiveRedisMessageListenerContainer) however it comes at a cost as it internally creates a new ReactiveRedisMessageListenerContainer instance on each invocation which can lead to excessive consumption of direct memory. This is somewhat related to issues like these:

• Getting java.lang.OutOfMemoryError: Direct buffer memory redis/lettuce#705
• Spring Reactive App backed by netty4 requires more direct memory cloudfoundry/java-buildpack-memory-calculator#20

I believe javadoc of those methods could warn users about resource usage and hint at considering whether multiplexed solution (in form of user managed ReactiveRedisMessageListenerContainer) is a more appropriate choice for their use case.