Provide a condition for checkpoint-restore

[wilkinsona]

We have a need for only defining beans when an application is using checkpoint-restore. We'd like to encapsulate this in a condition. Initially it will be equivalent to @ConditionalOnClass("org.crac.Resource") but this may change in the future. For example, we may need to introduce a property to make functionality opt-in if coordinated restore at checkpoint becomes part of the standard JDK.