Fallback to cache when connection is lost

[hannesrichter]

When using a proxy pool, the runtime should enable an optional fallback to locally cached objects for uninterrupted execution if the network connection to the API is lost.

[jonau]

there are a few questions we have to clarify first:

1. How do we detect that we don't have a network connection. I think it is only detectable either by a refused connection or a timeout (would that be configurable?).
2. The last option would mean, that we always need to wait for a possible network response, which would slow down the execution. Is this acceptable?

Maybe we should introduce a "offline mode" that is either activated by a connection error, or by the configuration. We try to reconnect at each request(except when it we force the offline mode) but immediately return the cached object. As soon as we get a valid response we switch back to "normal mode"