Better handling of trim/benchmark results.

Request benchmarking of devices once per week during normal fstrim
maintenance window.  Tunable parameter gives us the ability to change
frequency through global setting.

Track individual benchmark and trim results for each volume, and
use scrubbed volume identifier (based on adoptiong age) when logging
stats to drop box.  Track last benchmark and trim time for each
volume separately.

Bug: 21831325
Change-Id: I53b3ed788f7820c2e5bceb2840339f5b4aada3f0