Caches are currently shareable across sites and versions which can easily lead to ambiguous issues

Eloy and myself have been chatting about an issue that presently exists within MUC although we are not sure whether anyone has been affected yet or not.
The issue stands that cache keys don't contain anything relating to the site or the version it is running.
Because of this you are able to share caches between sites... which is great, providing the cache is "generic" like strings.
Otherwise if anything about the data being stored is unique to the site or version things can go horribly wrong.
Think questions being cached and database meta data.

We've come up with a plan to address this in the following way:
1. Add an option for cache definitions to specify the sharing options it supports.
2. Add an option for store instances allowing users to select which of the supported options is used.
3. Default to the strictest option by default which will always be siteid.

Also worth pointing out that this is a potential security concern if we continue to add caches to stable branches so must be backported.