Ask for more providers if all of them replied with DontHave

When all peers replied with `DontHave`, `BitswapBehaviour` should create `FindProviders` event. This event should be used from the upper layer for find which peers the specified CID and connect to them. On connection, `BitswapBehaviour` will ask the new peers for the CID automatically.

Discovery can be done via Kademlia, but this is a responsibility of the upper layer, not `BitswapBehaviour`.

Start by tracing BitswapBehaviour's handling of peer DontHave responses and its event definitions. Implement the FindProviders event for the case where all peers respond with DontHave, leaving provider discovery and connection handling to the upper layer. Done means the upper layer can receive this event with the requested CID and use it to find and connect to providers.

Written by the indexing model from the issue text.