Fixed OpenTSDB bug of incorrect downsampling unit while computing start and end time of HBase scan.
- Fixed the math. - Suffixed the field name with _ms, which means milliseconds
in the OpenTSDB convention. - Added a nested helper class for unit tests to
access private methods. - Added unit tests to address the bug.   * unit tests:
downsampleMilliseconds, and downsample - Factored out the unit tests for
downsampling from the huge TestTsdbQuery.java.