InfluxDBStore: use Continuous Queries to make the Dashboard very responsive
InfluxDBStore: use Continuous Queries to make the Dashboard very responsive

This change uses Continuous Queries to downsample our span data in such
a way as to make the Dashboard page very responsive even when there are
a large number of spans being stored.

Fixes #141

bg451/appdash,bg451/appdash,bg451/appdash