Return cached packets first before fetching from the network in the background

[Nuhvi]

Currently Pkarr clients only return cached packets if it failed to find anything on the DHT/resolvers/relay which means that you have to wait for the request_timeout (usually 2 seconds) before returning cached packets... that has proved to be wasteful in the case of cached packets with no-one to republish them.

The proposed change is to always return cached packets, and do the query for expired packets (in the background).

Note: this is the behavior of Resolvers already.

Pros: calling Client::resolve only takes longer if the client doesn't have any cached packets.

Cons: users will get outdated packets more often.