Add a nanosecond API to Time/Timer/FakeTimer

Right now Time/Timer export functionality for retrieving time at a millisecond-level precision but not at a nanosecond-level precision, which is required for some applications (there's ~70 usages). Most of these seem not to need mocking functionality for tests; only one class currently mocks this out (LightWeightCache) but we would like to add another as part of HDFS-11615 and want to avoid code duplication. This could be useful for other classes in the future as well.