Tests for ThrottleService, NTP into TrustedTime.

Wrote initial suite of tests for ThrottleService, checking a variety
of edge cases.  Checks going over limits, updating policies, and reset
after cycle elapses.

Moved NTP code in ThrottleService into new TrustedTime interface,
which makes it easier to understand, and allows tests to provide custom
clocks.

Change-Id: I0d62b8b3a169516a2ab2d33025f6fe30dc792be8