The possible actions a client can execute when determining to serve a stored response.
The stored response is fresh.
The cache must serve the stored response without validation.
A stored response was found, but is stale.
The cache must serve the stored response immediately, and validate in the background.
If the validation has not happened after now + delta seconds, then the resource should NOT
be served stale.
The cache must validate the request with the origin server.
If the origin server cannot be reached, then the cache MAY serve the
stale response. In addition, if staleOnError is true, then the cache can serve
the stale response when the origin server returns a 5xx response.
If the origin server cannot be reached, then the cache MUST serve a gateway timeout response.