storage: drop Len from the Batch interface

It's almost meaningless now and we can easily live without it.

pkg/core/blockchain.go

@@ -238,6 +238,7 @@ func (bc *Blockchain) AddHeaders(headers ...*Header) (err error) {
 	)
 
 	bc.headersOp <- func(headerList *HeaderHashList) {
+		oldlen := headerList.Len()
 		for _, h := range headers {
 			if int(h.Index-1) >= headerList.Len() {
 				err = fmt.Errorf(
@@ -258,7 +259,7 @@ func (bc *Blockchain) AddHeaders(headers ...*Header) (err error) {
 			}
 		}
 
-		if batch.Len() > 0 {
+		if oldlen != headerList.Len() {
 			if err = bc.memStore.PutBatch(batch); err != nil {
 				return
 			}

pkg/core/storage/memory_store.go

@@ -37,11 +37,6 @@ func (b *MemoryBatch) Delete(k []byte) {
 	b.del[kcopy] = true
 }
 
-// Len implements the Batch interface.
-func (b *MemoryBatch) Len() int {
-	return len(b.m)
-}
-
 // NewMemoryStore creates a new MemoryStore object.
 func NewMemoryStore() *MemoryStore {
 	return &MemoryStore{

pkg/core/storage/store.go

@@ -46,7 +46,6 @@ type (
 	Batch interface {
 		Delete(k []byte)
 		Put(k, v []byte)
-		Len() int
 	}
 
 	// KeyPrefix is a constant byte added as a prefix for each key