Rotate actor key once a day by default, instead of once an hour

I'm not sure what the best balance is, but once an hour may end up causing a
lot more key re-fetch requests coming from other servers. I prefer to default
to once a day for now (maybe even once a week) and tighten it later if needed.

Caveat: If an instance key is rotated once a day, there's no
change-key-right-after-toot-deletion thing for deniability. Potentially,
rotation may happen only 24 hours after that deletion, which is much more than
1 hour. On the other hand, it's a whole instance key, not personal key of the
actor.

config/settings-default.yaml

@@ -42,7 +42,7 @@ request-time-limit:
 # How often to generate a new actor key for HTTP-signing requests
 actor-key-rotation:
   amount:               1
-  unit:                 hours
+  unit:                 days
 
 ###############################################################################
 # Development