cmd/coordinator, cmd/buildstats: start of using logs to schedule tests

We've been logging event spans to datastore for years, but I'd lost
this CL and just found it back. This does two things: syncs the
datastore logs to BigQuery, and starts to use the from-BigQuery timing
info in the coordinator for scheduling sharded tests.

The plan was to have a job occasionally do a BigQuery query and write
out the results to a CSV file on GCS. The code to read that CSV file
is in this CL, but that code path is disabled, so this CL should be a
no-op.

A future change will periodically do the query and write the CSV file,
and then we can start using the new code path and remove the static
map of expected test durations.

Updates golang/go#12669

Change-Id: Ibe5b41d6a3009c2ade8ab728fa1cad646788e621
Reviewed-on: https://go-review.googlesource.com/30716
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed