Load images and storage references in bulk during V1 synthesize

Currently, we perform multiple queries for each layer, making it much slower (especially cross-region)

Fixes #413

data/model/storage.py

@@ -232,5 +232,10 @@ def get_layer_path(storage_record):
 
   return store.blob_path(storage_record.checksum)
 
-
-
+def lookup_repo_storages_by_checksum(repo, checksums):
+  """ Looks up repository storages (without placements) matching the given repository
+      and checksum. """
+  return (ImageStorage
+          .select()
+          .join(Image)
+          .where(Image.repository == repo, ImageStorage.checksum << checksums))