blockchain: Add tfFresh to txoFlags

This change is part of the effort to add utxocache support to btcd.

The fresh flag indicates that the entry is fresh and that the parent
view (the database) hasn't yet seen the entry.  This is very useful as
a performance optimization for the utxo cache as if a fresh entry is
spent, we can simply remove it from the cache and don't bother trying to
delete it from the database.

blockchain/utxoviewpoint.go

@@ -28,6 +28,13 @@ const (
 	// tfModified indicates that a txout has been modified since it was
 	// loaded.
 	tfModified
+
+	// tfFresh indicates that the entry is fresh.  This means that the parent
+	// view never saw this entry.  Note that tfFresh is a performance
+	// optimization with which we can erase entries that are fully spent if we
+	// know we do not need to commit them.  It is always safe to not mark
+	// tfFresh if that condition is not guaranteed.
+	tfFresh
 )
 
 // UtxoEntry houses details about an individual transaction output in a utxo
@@ -58,6 +65,12 @@ func (entry *UtxoEntry) isModified() bool {
 	return entry.packedFlags&tfModified == tfModified
 }
 
+// isFresh returns whether or not it's certain the output has never previously
+// been stored in the database.
+func (entry *UtxoEntry) isFresh() bool {
+	return entry.packedFlags&tfFresh == tfFresh
+}
+
 // IsCoinBase returns whether or not the output was contained in a coinbase
 // transaction.
 func (entry *UtxoEntry) IsCoinBase() bool {
@@ -199,7 +212,7 @@ func (view *UtxoViewpoint) addTxOut(outpoint wire.OutPoint, txOut *wire.TxOut, i
 	entry.amount = txOut.Value
 	entry.pkScript = txOut.PkScript
 	entry.blockHeight = blockHeight
-	entry.packedFlags = tfModified
+	entry.packedFlags = tfFresh | tfModified
 	if isCoinBase {
 		entry.packedFlags |= tfCoinBase
 	}