Implement ExternalBlock Type and Cache

[alanorwick]

ExternalBlocks are blocks that are sent from the manager with external transactions terminating in the receiving nodes context. We break down a standard block into an ExternalBlock in order to save space on disk for validating and time downloading from manager.

An ExternalBlock struct will look roughly like this:

// ExternalBlock represents an external block with transactions destined for a specific context.
type ExternalBlock struct {
	header       *Header
	externalTxs  Transactions
	originLocation byte
	
	// caches
	hash atomic.Value
	size atomic.Value

	// Td is used by package core to store the total difficulty
	// of the chain up to and including the block.
	td *big.Int

	// These fields are used by package eth to track
	// inter-peer block relay.
	ReceivedAt   time.Time
	ReceivedFrom interface{}
}

Implementation requirements:

• External Block cache of type *lru.Cache for fast look up during validation.
• Method of listening for blocks from manager to update the cache.