Add a placeholder instead of the logs when logs get rotated out

[Ekleog]

So that NixOS/ofborg-viewer#18 can then be done on the viewer's side.

[Ekleog]

       ekleog │ would just replacing the logs by "***DELETED LOGS***" during GC somehow be easily possible?
       ekleog │ (that's a bit of a hack, though)
 gchristensen │ strictly speaking, ideally, they'd just get relocated somewhere else for long term storage
MichaelRaskin │ And to save inodes, it is a hardlink to a single file with that line
       ekleog │ I'd say having something that works now sometime trumps having the ideal result, so maybe this hack and then when a long-term storage solution is found move
              │ the logs to the long-term storage?
       ekleog │ (that said there's no real emergency for this kind of issue, so maybe it's better to just leave it waiting for the long-term storage solution)
 gchristensen │ sometimes the hacks take longer than the long term solution
[...]
       ekleog │ hmm, was thinking the deletion process could be `echo "***DELETED LOGS***" > logfile` instead of the likely-current `rm logfile`, but I just can't find
              │ where this rotation code is implemented, so not sure at all :/