This cleans up tmp and logrotates /var/log every 12 hours.
Note that this janitor is separate from the box janitor because they
run as different users.
Fixes#503
We used to run this as a separate process but no amount of node/v8 tweaking
makes them run as standalone with 50M RSS.
Three solutions were considered for the memory issue:
1. Use systemd timer. apphealthtask needs to run quiet frequently (10 sec)
for the ui to get the app health update immediately after install.
2. Merge into box server (this commit)
3. Increase memory to 80M. This seems to make apphealthtask run as-is.
