Add Release & Error methods to Iterator

We need this to properly support LevelDB backend, which needs
to be released if we don't want to leak memory.

db/db.go

@@ -10,10 +10,10 @@ type DB interface {
 	DeleteSync([]byte)
 	Close()
 	NewBatch() Batch
+	Iterator() Iterator
 
 	// For debugging
 	Print()
-	Iterator() Iterator
 	Stats() map[string]string
 }
 
@@ -28,6 +28,9 @@ type Iterator interface {
 
 	Key() []byte
 	Value() []byte
+
+	Release()
+	Error() error
 }
 
 //-----------------------------------------------------------------------------

db/mem_db.go

@@ -97,6 +97,16 @@ func (it *memDBIterator) Value() []byte {
 	return it.db.Get(it.Key())
 }
 
+func (it *memDBIterator) Release() {
+	it.db = nil
+	it.keys = nil
+	return
+}
+
+func (it *memDBIterator) Error() error {
+	return nil
+}
+
 func (db *MemDB) Iterator() Iterator {
 	it := newMemDBIterator()
 	it.db = db