Release the list of chunk hashes after processing each snapshot.

The chunk hash list isn't needed any more after being consolidated.
Releasing it immediately after use helps reduce the memory usage.

src/duplicacy_backupmanager.go

@@ -1668,6 +1668,8 @@ func (manager *BackupManager) CopySnapshots(otherManager *BackupManager, snapsho
 				chunks[chunkHash] = true
 			}
 		}
+
+		snapshot.ChunkHashes = nil
 	}
 
 	otherChunkFiles, otherChunkSizes := otherManager.SnapshotManager.ListAllFiles(otherManager.storage, "chunks/")