query cache hits scramble timings

[favila]

When using the query cache and when a query is a cache "hit", the ::timing-start value that is used is the value from the initially-cached query, not from the current request.

The bug is this line, which returns the cached value unchanged instead of associng the "fresh" timing-start value passed in to the function.

As an additional wrinkle, the parsing timing info is also cached, so the value reported is really only relevant for the initial request. I'm not sure what to do here. A possibility is to time the cache lookup and regard that as the parse offset+duration if it's a "hit".

(Related because also a timing bug: walmartlabs/lacinia#448)

[hlship]

This does look like an issue, thanks for researching it so thoroughly.