Auto-configure a sensible TaskExecutor

[BigMichi1]

it would be really nice if spring-boot has an auto-configuration for the AsyncTaskExecutor. So if an @EnableAsync is found a AsyncTaskExecutor bean is provided based on the environment. If there is support for jsr236 (e.g. wildfly) the DefaultManagedTaskExecutor is used by looking in jndi and when not a ThreadPoolTaskExecutor or a SimpleAsyncTaskExecutor is used.

Perhaps this can be combined with the @EnableScheduling configuration

[philwebb]

See also #1397

[philwebb]

@snicoll Did we discuss this briefly already? any thoughts?

[snicoll]

I don't think we did but if we feel we should change Spring's own defaults we should just do it. This may require a larger discussion though (as mentioned by your link)