i2psnark standalone: Fixes for router startup and shutdown

so that torrents stop when the router stops and restart when the router restarts.

- Use BWLimits from the DirMonitor as a periodic test that the router is there
- DirMonitor does not attempt to autostart torrents if BWLimits test fails
- DirMonitor does autostart existing torrents when BWLimits test passes again
- Register disconnect listener with socket manger and stop all torrents on disconnect
- Use stopTorrent(true) on router errors to prevent changing the persisted torrent running status
- Change autostart default to true for standalone

Possibly more todo for corner cases or other start/stop/fail scenarios.