1. There is simpler way to express that 0 or 1 bean only is allowed
  2. This commit allows flexible AsyncConfigurer substitution with @Primary

Comment From: snicoll

Thanks for the PR but I'd like to keep things as they are. One gotcha is that the supplier is kept as late as possible and only resolved when necessary whereas this change can trigger early initialization.

I also agree they are simpler ways to express the 0-1 beans use case but I favor readability of the code vs. optimization for configuration arrangements. Thanks anyway