add Caffeine cache support

[ben-manes]

This is derived from the Guava module. The changes were,

• Replace Guava types with Caffeine's (Cache, LoadingCache, CaffeineSpec)
• Add AsyncLoadingCache support
• Update documentation, modules, etc.

Note that AsyncLoadingCache does not extend the Cache interface due to some minor differences. It would ideally be a LoadingCache<K, CompletableFuture<V>>, but that would require allowing a null return value (as Caffeine does not throw an exception on a cache miss). To support this type, doWithAsync was added along with AsyncCallback. The private callback instance has weaker type safety, but is internal so this has no negative effect. That approach allowed for making the most minimal changes from the original Guava code.

[coveralls]

Coverage decreased (-0.008%) to 99.824% when pulling b5acf6e on ben-manes:master into 32c4f06 on jooby-project:master.

[jknack]

move this file to md/doc/caffeine/caffeine.md. The release tool will create this file later.

[jknack]

looks great! just did some minor comments

[ben-manes]

Done. Added myself as secondary author, since your code was changed in only minor ways when adapting. I also just released a new minor version with an enhancement in a sibling module. That might cause a build break until Sonatype staging fully replicates to Central.

[coveralls]

Coverage decreased (-0.008%) to 99.824% when pulling 09288c8 on ben-manes:master into 32c4f06 on jooby-project:master.