listenbrainz: avoid blocking network connections and close connection after use

This was running a network call on the main thread, potentially blocking RB. As the submission is performed in intervals anyway this is not really needed, so just remove the initial call completely. Fixes #1818 (closed)

Also apply a network timeout to all requests so they don't potentially stay open forever (blocking the thread they are running in) and make sure to close the connection after use (fixes #1824 (closed) )