add an async way to add torrents

[milahu]

currently the "add torrent" api endpoint is blocking

• on fetching metadata from a magnet link
  • default timeout is 10 minutes, should be possible to set metadata_timeout=infinite - or is this my http client timeout? (python requests library)
• on checking existing files (todo verify)
• when another torrent is checking files

this is unexpected and can be inconvenient for scripting

possible solution: add an async=true get parameter to return immediately and queue the request in Add torrent through HTTP API

keywords

• adding torrent hangs when another torrent is checking

fix #297