Instead of storing the parsed response and then serializing and
deserializing on every cache hit, the cache can be simplified by storing
the original serial only, and deserializing on cache hit.
ETag cache decorator methods are given JSDocs to encode the fact that
response serials are stored rather than parsed objects. Tests are added
to encode _getChangeDetail's interactions with the decorator cache.
Bug: Issue 7169
Change-Id: I6398b7477957460a3c2fd9bdaf8fa3c8bfa6f22c