Download fetches from closest node

[Eknir]

Epic

Story

As a user of Swarm, I want to know to which node I should ask for delivery of a certain chunk, such that chunk that exists in the network is found by the node whom I ask.

Background

Global vs local closest

If chunk are stored by the node whose address is closest to the chunk (see #48), then the node should request chunk delivery from the node whose address is closest to the chunk. In a network with full connectivity, the node who is closest (local) is also the node who is closest (global), but when we implement partial connectivity, the decision to ask the closest (local) node should be a step in the direction towards the closest (global).

Incentivized protocol

The retrieval protocol was incentivized in the old Swarm. The incentives team made a post-mortem based on their work. For the acceptance criteria for this PR, it is not needed to be incentivized, but the ground should be made ready for this protocol to become incentivized.

Acceptance criteria

• In a network where n random chunks are stored at the nodes whose addresses are closest to the respective chunks (see Upload syncs to closest node #48), a node is able to download all n chunks with 1 attempt per chunk

Dependencies

• Adjust full topology ChunkPeer method to return closest peer
• Introduce notion of NetStore. Strong emphasis on how we are going to do caching
• Wire HTTP JSON API to try to retrieve chunk from localstore; when that fails - initiate network request through retrieval protocol, get the chunk, optionally store it then serve it to the requester