Allow I2PSnark to save and recall state between sessions

[Reportage]

Currently I2PSnark offers the option to autostart torrents at the beginning of the session. Offering the option to save and resume a session in the state it was prior to shutdown would offer much more fine-grained control.

A user may have a number of torrents loaded, some stopped, some seeding, some downloading, and wish to have that state preserved when the router/I2PSnark is shutdown. Allowing the option to save the torrent session state on shutdown and to resume on startup would therefore be helpful.

[zzz]

See #1635 and #1686 result of incomplete/buggy fix for autostart #766 by dg. He gave up and so did I trying to understand what he did. Not looked at in 2 years.