ApplicationContext ID generation

ApplicationContext ID generation is difficult in general. Cloud Foundry solves this problem for us by providing unique instance ids to all running instances of an app. I don't suppose that helps much in the general case though, and we need something (a rule of thumb) that is unique and preferably deterministic, so that nodes retain their ID across process and connector restarts.

In Cloud Foundry the instance id plays a vital role (and will be automatically picked up by the app and applied - need to look at how that plays in a context hierarchy). User can set the context id manually using spring.application.name and spring.application.index.