Don't be too chatty about periodic tasks taking a few ms

Right now any background task that takes 3 ms or longer will result in a log message. The lock manager cleanup may take a few ms to clean all 128 or so buckets (though it will release/reacquire mutexes between buckets), causing one log message per minute on a normally operating system.

So, we should only log operations taking more than 100ms or so.