Persist peer lists for all torrents optionally

[josecelano]

The live demo server restarts the tracker container every two hours because it consumes too much memory. Every time the tracker is restarted, all info about all torrents is lost except for the statistics (number of downloads) if it's enabled (option persistent_torrent_completed_stat in configuration).

Maybe we could persist the torrent repository data (all torrents with peer lists). That way, when the tracker restarts, it can load that info from the database. This could also be an optional feature.

I do not know if it makes sense because:

1. That would make the tracker slower. It could handle fewer requests. In fact, we disable all persistency for benchmarking.
2. After 2 minutes, the tracker has the same information again because peers should announce themselves every 2 minutes (default value).

[josecelano]

Relates to: #774 (comment)

I think the only way to implement this without decreasing performance is if we switch to a key DB database in the torrent repository implementation and that does not reduces performance.