storage: don't use locks for memory batches

They're inherently single-threaded, so locking makes no sense for them.

pkg/core/storage/memory_store.go

@@ -23,12 +23,12 @@ type MemoryBatch struct {
 
 // Put implements the Batch interface.
 func (b *MemoryBatch) Put(k, v []byte) {
-	_ = b.MemoryStore.Put(k, v)
+	b.MemoryStore.put(string(k), slice.Copy(v))
 }
 
 // Delete implements Batch interface.
 func (b *MemoryBatch) Delete(k []byte) {
-	_ = b.MemoryStore.Delete(k)
+	b.MemoryStore.drop(string(k))
 }
 
 // NewMemoryStore creates a new MemoryStore object.

pkg/core/storage/store.go

@@ -52,7 +52,8 @@ type (
 
 	// Batch represents an abstraction on top of batch operations.
 	// Each Store implementation is responsible of casting a Batch
-	// to its appropriate type.
+	// to its appropriate type. Batches can only be used in a single
+	// thread.
 	Batch interface {
 		Delete(k []byte)
 		Put(k, v []byte)