Fastlogger OOM's when setting log_backup_number to maxBound on logfile rotation

[jappeace]

Recently keter was moved over to fastlogger,
we set the LogFile limit conservatively to maxInt to make sure it doesn't lose information.

https://github.com/snoyberg/keter/blob/28ba9b538b9e3850922c5e59a8e3038425a11f9a/src/Keter/Logger.hs#L51

Now we got a bugreport that it crashes on any rotation.
snoyberg/keter#288

I'll work around this by setting it to 20.

[kazu-yamamoto]

I can remove reverse from rotate but n must not be huge anyway.