Summary:
Add MultiLevelTimeSeries class which represents a timeseries which keeps several
levels of data granularity (similar in principle to the loads reported by the
UNIX 'uptime' command).  It uses several instances (one per level) of
BucketedTimeSeries as the underlying storage.

This can easily be used to track sums (and thus rates or averages) over several
predetermined time periods, as well as all-time sums.  For example, you would
use to it to track query rate or response speed over the last 5, 15, 30, and 60
minutes.

Test Plan: unittest included.

Reviewed By: simpkins@fb.com

FB internal diff: D851444