do a delta diff instead of updating resources extraneously

at the moment chalice on execution will always update resources instead of checking to see if even needs to update resources.

at the moment chalice fetches the info for current resource state and throws it away/casts it to return a boolean for resource existence.

No files or tests are named. Start by tracing Chalice's execution path that fetches the current resource state and currently discards it after checking resource existence. Define the desired-versus-current comparison and verify that execution updates only resources with differences, while leaving unchanged resources untouched.

Written by the indexing model from the issue text.