Don't allow a bloated job to permanently keep a worker's memory usage high.
Don't check memory after every job.

* From my benchmarks, it takes about 3-4ms to get the
  current memory usage since we shell out since ruby
  doesn't provide us with a system call API for that.
* I don't want to add 3ms to the time it takes to run
  every job.  Adding 3ms to every 10th job isn't so
  bad, though.