Consider how to support spring.integration.keep-alive

[philwebb]

Spring Integration is introducing a new spring.integration.keep-alive property which will create a keep-alive thread based on this logic. We could surface this property in Spring Boot, however, we already have spring.main.keep-alive.

We need to consider what we should do by default, specifically:

• If spring.main.keep-alive is true then there is no need to Spring Integration to create a thread
• If spring.main.keep-alive is missing then standard Spring Integration logic should apply
• If spring.main.keep-aliveis false we may need to also disable Spring Integration keep alive
• If we don't do the above, we need to offer a way to turn off Spring Integration keep alive threads

[philwebb]

See spring-projects/spring-integration#9455 for the original request. We need some more time to consider our options portfolio wide so we'll need to look at this after 3.4