export() does not work correctly in policies included in the local chain

[davidor]

Policies can define an export function to expose the data they need to share with other policies.

export works fine for policies defined in the global chain, but it does not for policies include in a local (service level) chain. Whatever is returned in export does not end in context.

When this bug is solved, we could also solve #671 by simply using export() instead of rewrite() in the caching policy to expose the shared cache handler.