InfluxDBStore: use Continuous Queries to make the Dashboard very responsive

[emidoots]

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

Based on #157 for simplicity.

[keegancsmith]

Very fancy! InfluxDB looks pretty damn cool. Two questions:

• Can this change have any test coverage (or is it covered by existing tests)
• What happens if no events happen in a minute? (ie does everything work as expected)

Otherwise code and idea LGTM

[emidoots]

These are good questions.

Can this change have any test coverage (or is it covered by existing tests)

Currently there are no tests for the Aggregate method at all, and I agree we should add them. For this I've filed #160 in an effort to not block this PR. I have done extensive testing locally and everything does work, so I'm confident this is a clear improvement. Let's invest in these tests soon.

What happens if no events happen in a minute? (ie does everything work as expected)

I did test this case locally as well, and can confirm it does. In specific, InfluxDB handles any query on a non-existing measurement (table) by returning no results, thus hitting this case https://github.com/sourcegraph/appdash/pull/158/files#diff-b7da646763ca1f5622f8d108c536ce86R295

[emidoots]

Merged into master.