chainntnfs: add heightHint to RegisterSpendNtfn+ RegisterConfirmationsNtfn

This commit modifies two of the main methods in the ChainNotifier
interface to be more light client friendly. In order to do so, we now
tack on an extra parameter to the methods: heightHint. This value
represents the earliest known height that the chain should be scanned
when attempting to do a dispatch from historical data.

All tests have also been updated to use these new parameters properly
when excising the expected behavior of each interface implementation.

chainntnfs/btcdnotify/btcd.go

@@ -618,7 +618,8 @@ type spendCancel struct {
 // outpoint has been spent by a transaction on-chain. Once a spend of the target
 // outpoint has been detected, the details of the spending event will be sent
 // across the 'Spend' channel.
-func (b *BtcdNotifier) RegisterSpendNtfn(outpoint *wire.OutPoint) (*chainntnfs.SpendEvent, error) {
+func (b *BtcdNotifier) RegisterSpendNtfn(outpoint *wire.OutPoint,
+	_ uint32) (*chainntnfs.SpendEvent, error) {
 
 	if err := b.chainConn.NotifySpent([]*wire.OutPoint{outpoint}); err != nil {
 		return nil, err
@@ -694,7 +695,7 @@ type confirmationsNotification struct {
 // which will be triggered once the txid reaches numConfs number of
 // confirmations.
 func (b *BtcdNotifier) RegisterConfirmationsNtfn(txid *chainhash.Hash,
-	numConfs uint32) (*chainntnfs.ConfirmationEvent, error) {
+	numConfs, _ uint32) (*chainntnfs.ConfirmationEvent, error) {
 
 	ntfn := &confirmationsNotification{
 		txid:             txid,