Log file should rotate out when it hits its size limit, not just stop recording

[QuintillusCFC]

Right now, if Serilog hits its file size limit (configured in LogManager.cs), it just stops recording.

I would have expected it to remove old logs but keep recording, or create a second log file, or something, but not to stop recording.

This is to fix that.

My current thoughts on the ideal are:

• Log file per session of running the game. Keep up to 5 or 10 of them. That way it's possible to send us logs for a bug from a recent, but not most-recent, game session
• If you run a session long enough that it hits the file size limit, ideally it should drop the old logs and start anew. My ideal would be it drops, say, half of the old logs (rather than 100%), but anything is better than not recording any more logs.

[WildWeazel]

relates to #259