Implement Block Format for Blockstore writes

[dutterbutter]

Blocks written to the blockstore are stored in a particular format consistent with IPFS blocks. When we are informed of new blocks that need to be stored we need to format them accordingly prior to writing. The format is used prior to storing blocks and messages into blockstore.

Go Library of Block Format:

• go-block-format

See usage examples in implementations here:

• Lotus- PersistBlock
• Lotus- ToStorageBlock
• Lotus- ToStorageBlockMsg

Block Format is defined:

type BasicBlock struct {
	cid  cid.Cid
	data []byte
}