Implement NewBlockListGrowthPolicy (#35)

Right now this is unimplemented, which renders LocalBlobAccess unusable in
bb-remote-asset!  Worse still, the UX is an error message that reads like the
fault is on the part of the user.  Let's implement the same policy as in
ProtoBlobAccessCreator (as this stores protos, like the Action Cache).

pkg/storage/blobstore/asset_blob_access_creator.go

@@ -57,8 +57,11 @@ func (bac *assetBlobAccessCreator) GetDefaultCapabilitiesProvider() capabilities
 	})
 }
 
-func (bac *assetBlobAccessCreator) NewBlockListGrowthPolicy(int, int) (local.BlockListGrowthPolicy, error) {
-	return nil, status.Error(codes.Unimplemented, "NewBlockListGrowthPolicy unimplemeted for assetBlobAccessCreator")
+func (bac *assetBlobAccessCreator) NewBlockListGrowthPolicy(currentBlocks, newBlocks int) (local.BlockListGrowthPolicy, error) {
+	if newBlocks != 1 {
+		return nil, status.Error(codes.InvalidArgument, "The number of \"new\" blocks must be set to 1 for this storage type, as objects cannot be updated reliably otherwise")
+	}
+	return local.NewMutableBlockListGrowthPolicy(currentBlocks), nil
 }
 
 func (bac *assetBlobAccessCreator) NewHierarchicalInstanceNamesLocalBlobAccess(local.KeyLocationMap, local.LocationBlobMap, *sync.RWMutex) (blobstore.BlobAccess, error) {