Use Blob Serialization to Construct Collation Instances

[rauljordan]

We can resolve some TODO's by taking advantage of the blob serialization algorithm created in #92 by @nisdas. When creating a new collation object and having access to the collation's byte serialized body, we can deserialize that body into a slice of transactions that we can pass into the NewCollation function.

An example of this is in shard.go, but there are other similar todo's.

// CollationByHash fetches full collation.
func (s *Shard) CollationByHash(headerHash *common.Hash) (*Collation, error) {
	header, err := s.HeaderByHash(headerHash)
	if err != nil {
		return nil, fmt.Errorf("cannot fetch header by hash: %v", err)
	}

	body, err := s.BodyByChunkRoot(header.ChunkRoot())
	if err != nil {
		return nil, fmt.Errorf("cannot fetch body by chunk root: %v", err)
	}
	// TODO: deserializes the body into a txs slice instead of using
	// nil as the third arg to MakeCollation.
	col := NewCollation(header, body, nil)
	return col, nil
}

Another one occurs when saving the collation's body:

// SaveBody adds the collation body to the shardDB and sets availability.
func (s *Shard) SaveBody(body []byte) error {
	// TODO: check if body is empty and throw error.
	// TODO: dependent on blob serialization.
	// right now we will just take the raw keccak256 of the body until #92 is merged.
	chunkRoot := common.BytesToHash(body)
	s.SetAvailability(&chunkRoot, true)
	return s.shardDB.Put(chunkRoot, body)
}

[nisdas]

Assigned to Eli