Fix incorrect downsampling unit

The bug incorrectly assumed the downsampling unit was second while computing
start and end time of HBase scan.
- Fixed the math.
- Suffixed the field name with _ms.
- Added unit tests to address the bug.
  * downsample and downsampleMilliseconds.
- Added a nested helper class for unit tests to access private methods.
- Factored out the unit tests related to downsampling from the huge
  TestTsdbQuery.java. It was too big for Eclipse to run.