Keep to serve BZZ cached data even if node is disconnected by network

[tmm360]

Summary

When a Bee node disconnect for some reason from kademlia network, it should be able to reply to GET requests on BZZ api for cached data. Currently it closes APIs, but because BZZ is immutable, if they are cached they could be returned.

Motivation

This improve node resiliency, even with network's issues.

Implementation

API should stay always accessible, and response according to data availability.

Drawbacks

Don't see any.