state: clone params.StateServingInfo

This PR breaks a dependency of the `state` package on the `apiserver/params` package. The type in question was `params.StateServingInfo` which has been replicated into the `state` package as unfortunately this API type has been used to describe a document in mongodb.

Most of the uses of `params.StateServingInfo` have been replaced with `state.StateServingInfo` and there is a helper method in the `agent` package to translate between the types for the API.