Add ETL active updates and heartbeats.

[gfr10598]

Part of m-lab/dev-tracker#501

Use gardener update/ to send per task updates. Use gardener heartbeat/ to send per job heartbeat, once per minute. This allows gardener to detect ETL instance crashes.

[gfr10598]

Lots of prototyping in the universal-sandbox-gfr branch. Had stuff working with alpine-glibc, but Stephen suggested avoiding that dependency. However, with

FROM ubuntu
RUN apk add --no-cache ca-certificates

I see errors on the etl side like:

active.go:170: Error streaming Get https://storage.googleapis.com/storage/v1/b/archive-mlab-sandbox?alt=json&prettyPrint=false&projection=full: x509: certificate signed by unknown authority

Trying again with:

FROM frolvlad/alpine-glibc
RUN apk update && apk add ca-certificates && rm -rf /var/cache/apk/*