service/block: Implement block store

[renaynay]

This PR implements the block store which allows block data to be stored and retrieved via the provided DAGService. The block.Service receives an ipld.DAGService upon construction and uses it for writing and reading block data. Block data can be retrieved via the DataAvailabilityHeader.

Merging blocked by #110.

Resolves #47.
Related to #25.

[renaynay]

A question this PR raises is how to best make the most recent (arbitrary number) number of blocks quickly available for sampling as those are most likely going to be sampled from the most following a new block event in the network. Do we store the block data in mem for a while and then pop it out after X number of new blocks?

[mattdf]

Do we store the block data in mem for a while and then pop it out after X number of new blocks?

This sounds reasonable to me, perhaps do what geth does and allow the user to specify a command line argument that sets the max RAM allowed to be used by the cache, so that very big blocks don't DoS nodes in the future. If a node has to restart for whatever reason, then you'd also have to rebuild this cache by reading the disk. I think some kind of ring buffer struct would be fine here since for sampling you'll want to access the entire block.

Current PR LGTM.

[liamsi]

👍🏼

[liamsi]

☀️🕺🏻🎉🍾