The server keeps a play queue per client, identified by the X-Queue-Id
header. The client sends the ID it has for the selected server with every
request and keeps the one in each response, which is new after a reset or a
sync. The IDs are saved per server in the [queues] section of settings.cfg,
so the game goes on with the same queue after a restart.
HttpTransport now passes the response headers on to the client.
HTTPRequest has no timeout by default, so a server that accepts the
connection but never responds left song downloads waiting forever.
Requests now fail after 120 seconds, which triggers the normal retry.