USe parallel limit on zip file streaming

services/web/app/coffee/Features/Downloads/ProjectZipStreamManager.coffee

@@ -48,4 +48,4 @@ module.exports = ProjectZipStreamManager =
 							archive.append stream, name: path
 							stream.on "end", () ->
 								callback()
-			async.series jobs, callback
+			async.parallelLimit jobs, 5, callback