Bucket size returned with timeseries responses

Timeseries responses are now self-describing with regard to their bucketing.

1. Every timeseries point now carries a bs (bucket size) field reflecting the granularity chosen for that query.
2. Mixed-granularity points are coalesced safely on the server side.
3. The 60-minute and 6-hour bucket options were removed from the public timespan menu, as they produced aliasing artifacts on shorter ranges.
4. Improved handling of null timestamp cases.