Improve caching of blob redirect responses

[pditommaso]

When serving container requests Wave behave as a proxy in between the Docker client and the target container registry.

This means that to serve requests it's needed to connect the target registry, and therefore the quality of the service may be affected by the reliability of the target registry.

To improve the service throughput all redirect responses from the target registry can be cached by wave to avoid to avoid reaching the remote service once the response has been collected.

This goal can be achieved caching the result of handleRequest, when:

• the response status is a redirect code
• or, when it's returned a 2xx..3xx http status and the response body is available