Implement reading from IPLD merkle dag

[liamsi]

Summary

The methods that read from IPLD as drafted in #170 / adr 002 need to be implemented too.

Problem Definition

We need a simple API that can:

• sample a given number of chunks
• reconstruct the whole block

Proposal

Implement the methods ValidateAvailability, RetrieveBlockData (and GetLeafData). Given that this will essentially replace the block store, we should create some lightweight "DagGetter" object that provides the first two methods.

ref: #190
ref: https://github.com/lazyledger/lazyledger-core/blob/master/docs/lazy-adr/adr-001-block-propagation.md#disk-storage

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