Simplify getCache() method in CaffeineCacheManager

[bananayong]

I would like to make some contributions.

Current spring framework is based on JDK8. So, CaffeineCacheManager.getCache() method can be simplified with Map.computeIfAbsent

Thank you.

[pivotal-issuemaster]

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[sbrannen]

Thanks for the PR!

I ended up implementing this slightly differently in 16e49bf.

[bananayong]

Thank you to reflect my contribution.
It looks simpler!

But, I would like to give an opinion.

cacheName -> this.dynamic ? createCaffeineCache(cacheName) : null

This lambda expression accesses this.dynamic member field. It causes capturing variable.
getCache method will create new lambda instance whenever called.
It can be avoided if this.dynamic is checked outside lambda.

Please, ignore this comment if there is no big benefits.
Thank you!