blockchain: change nodesbackend deletion

For NodesBackend, the cached entry is not removed from the cache even
if it's being deleted as subsiquent calls would be made to fetch the
key. Caching it as removed saves on disk reads and on flushes, the leaf
is not attempted to be flushed if it's marked as fresh.

blockchain/utreexoio.go

@@ -155,6 +155,9 @@ func NodesBackendDelete(tx *leveldb.Transaction, k uint64) error {
 // Delete removes the given key from the underlying map. No-op if the key
 // doesn't exist.
 func (m *NodesBackEnd) Delete(k uint64) {
+	// Don't delete as the same key may get called to be removed multiple times.
+	// Cache it as removed so that we don't call expensive flushes on keys that
+	// are not in the database.
 	leaf, _ := m.cache.Get(k)
 	l := utreexobackends.CachedLeaf{
 		Leaf:  leaf.Leaf,
@@ -254,7 +257,7 @@ func (m *NodesBackEnd) UsageStats() (int64, int64) {
 // flush saves all the cached entries to disk and resets the cache map.
 func (m *NodesBackEnd) Flush(ldbTx *leveldb.Transaction) error {
 	err := m.cache.ForEach(func(k uint64, v utreexobackends.CachedLeaf) error {
-		if v.IsRemoved() {
+		if v.IsRemoved() && !v.IsFresh() {
 			err := NodesBackendDelete(ldbTx, k)
 			if err != nil {
 				return err