Commit to release v0.5.0:
- Major algorithm changes:
  - `AbstractCounter.getSeries(...)`: `timestampStartMs` and
`timestampEndMs` are no-longer rounded, caller is responsible for
providing correct timestamp-range.
  - `AbstractCounter.getLastN(...)`: update rounding formula to cope
with change(s) in method `getSeries(...)`.
- `CassandraCounter`:
  - (Experimental) support `set` operator for counter columns.
  - (Experimental) support `add` operator for bigint columns.