DBC: Implement in-memory MerkleReg backed OrderingDAG

[joshuef]

Where

Probably inside sn/data/spentbook

What

We need the following API's implemented on top of the Ordering DAG (which in our case will simply be the MerkleReg)

type OrderingDAG = MerkleReg<MaidTransaction>;
type InvertedDAG = BTreeMap<Hash, BTreeSet<Hash>>; // used for bootstrapping and traversing the DAG forward

struct MaidTransaction {
    // hash of the MaidTransaction that produced an output with this inputs public key
    inputs: BTreeMap<PublicKey, Hash>,
    ringct: RingCtTransaction, // The actual transaction
}

/// return the hashes of the most recent MaidTransactions
fn spentbook_roots(&self) -> BTreeSet<Hash>

/// return the MaidTransaction for the given hash
fn spentbook_read(&self, tx_hash: Hash) -> Option<MaidTransaction>

/// returns the transactions that came after the transaction with hash `tx_hash`
/// This will be used to traverse the SpentBook from the genesis onwards.
fn spentbook_next_transactions(&self, tx_hash: Hash) -> Option<BTreeSet<Hash>>

The in memory dag would only be written to as/when we have log_spent in place.

This is just a quick issue to get the basics of a spent book in place.