Analysis on changes to instance properties during an upgrade

When we upgrade Sleeper, this can introduce new CDK-defined properties, or changes to their values. These aren't written to the S3 bucket until the end of a CDK deployment.

Every process deployed by the CDK will start as soon as it is deployed. They won't see the new values of the CDK-defined properties until the end of the deployment, and then only if the process reloads the instance properties.

Similarly in cases where old processes continue past the point where the new instance properties are written, if they reload the properties they'll see the new version. We need that to be compatible.

We'd like to document these concerns and what we can do to check we won't have problems.