Make housekeeping interval dynamic, fist signal is resource usage.

If no resources are used by the container since the last housekeeping,
we double the housekeeping interval until --max_housekeeping_interval.
If usage was detected, we drop it back to the baseline
(--housekeeping_interval).

From my tests this reduces CPU usage in CoreOS from ~8% to ~3% with no
real loss of accuracy.

Fixes #159
Merge pull request #223 from vmarmol/dynamic-housekeeping

Make housekeeping interval dynamic, fist signal is resource usage.