Add a config option to save the metrics data locally.

Usually after config owner calls statsd to get data, statsd will remove that data
forever. This cl adds an option to save historical data locally so that when a bugreport
is taken, it contains historical statsd metrics for debugging.

+ All existing guardrail on storage still apply, and local history data has lower
  priority when the guardrail is hit. On top of that, statsd will remove history
  files that are more than 2 days old.

+ Also fixed issues inside StorageManager (e.g.,
   using string::compare to sort by timestamp is problematic, redundant file size read, etc)

Test: statsd_test and manually
Fix: 126562025

Change-Id: I5238b9fd3069cbef5bcccabd2212bc4d6c734702