Logging: rollover log files

[pacien]

We currently rely on the system for clearing all log files when their size becomes problematic. It would be nice to manage the amount of log history that we want to keep instead.

For the Android application itself, this is straightforward: we just need to swap Logback's FileAppender with RollingFileAppender and define some cap for the history length.

For the tinc daemon, this is more involved. We could make the daemon use a new log file each time it starts and delete the older ones at each start.