Parallelize delta generation across partition

On my machine, this change alone reduces incremental
OTA generation time from 56 minutes to 29 minutes

Test: Generate and serve an OTA
Change-Id: Id4ffc6f02f28594eb60cb934777b82f1899bbbc2
diff --git a/payload_generator/payload_file.h b/payload_generator/payload_file.h
index 9dc80a7..0c3e9d5 100644
--- a/payload_generator/payload_file.h
+++ b/payload_generator/payload_file.h
@@ -43,7 +43,7 @@
   // reference a blob stored in the file provided to WritePayload().
   bool AddPartition(const PartitionConfig& old_conf,
                     const PartitionConfig& new_conf,
-                    const std::vector<AnnotatedOperation>& aops);
+                    std::vector<AnnotatedOperation> aops);
 
   // Write the payload to the |payload_file| file. The operations reference
   // blobs in the |data_blobs_path| file and the blobs will be reordered in the