WIP - actively ingest measurements

This work goes hand in hand with what was done with `sensord`.  Instead of having `sensord` instead `POST /measurements`, were going to stream them in over chunked http.  This helps reach some of the goals of https://github.com/canaryio/meta/issues/2, but allows us to forget about introduce a new component for now.

Prior art:

* https://github.com/canaryio/sensord/pull/42
* https://github.com/canaryio/ingestd