Expose the underlying cache infrastructure bean if Boot auto-configures
it. This is the case for ehCache, hazelcast and JCache. This change has
two side effects:

1. It is now possible to customize the underlying cache infrastructure
and let Boot only wrap it in the Spring's CacheManager abstraction. No
customizations are applied if the caching-specific service is customized
2. Such infrastructure is disposed when the application terminates as
it is now defined as `@Bean` and both `close()` and `shutdown()` methods
are invoked if present on the target type.

While the latter can be troublesome, we feel that a particular cache
instance is not meant to be shared and must be disposed when the
application terminates.

Closes gh-2848