parallelize fetching of remote scripts
Besides scripts keps in extension's storage, it is also possible to define an injectable script using URL from which it can be downloaded and an SHA256 hash to verify its integrity. Code that handles such downloads is kept in background/page_actions_server.js. As of now, downloads are performed sequentially. We can optimize them to happen in parallel instead.
Care must be taken to later inject the scripts in the right order and to avoid races or duplication of scripts (yes, repeats are possible). Also, this feature hasn't been tested anytime recently. Be ready to encounter bugs with current code.