housekeeping log messages are noisy

[garlick]

Problem: consider dropping these log messages if no longer useful, as they can be pretty noisy on a system that runs a lot of jobs.

[ +21.563248] job-manager[0]: housekeeping: fBFD1VEyMzw started
[Oct07 05:20] job-manager[0]: housekeeping: fBFCz2MSV7m complete
[  +0.115570] job-manager[0]: housekeeping: fBFCzPir8i3 complete
[  +2.710637] job-manager[0]: housekeeping: fBFD18LFVUs complete
[  +5.504068] job-manager[0]: housekeeping: fBFD1VEyMzw complete
[  +6.689593] job-manager[0]: housekeeping: fBFD5aUUSTH started
[ +41.918286] job-manager[0]: housekeeping: fBFD5aUUSTH complete
[Oct07 06:13] job-manager[0]: housekeeping: fBFD5QCVRAf started
[ +34.514484] job-manager[0]: housekeeping: fBFD5QCVRAf complete

[grondo]

These messages are still somewhat useful IMO because there is no other way to determine how long a housekeeping job took after the fact. I can see how they do take up a lot of space in the logs, though.

A couple random thoughts:

• it seems like common practice to keep log messages like this at the DEBUG level, then if you don't want noisy logs, just suppress DEBUG messages. Do we need to do that on larger systems or those with many jobs?
• maybe the housekeeping subsystem could add some stats like min/max/median runtime for jobs to fulfill the interest in knowing how long housekeeping is taking, if there was any problem runs, etc.