Add support for reading metadata from a file.
Add support for reading metadata from a file.

Introduce a "metadata_file" parameter that contains exactly the
records we want to fetch. Circumvents the S3-based client index
code.

Also implement proper "stop" behaviour.

whd/data-pipeline,sapohl/data-pipeline,nathwill/data-pipeline,nathwill/data-pipeline,whd/data-pipeline,mozilla-services/data-pipeline,nathwill/data-pipeline,whd/data-pipeline,mozilla-services/data-pipeline,mozilla-services/data-pipeline,whd/data-pipeline,acmiyaguchi/data-pipeline,acmiyaguchi/data-pipeline,acmiyaguchi/data-pipeline,sapohl/data-pipeline,acmiyaguchi/data-pipeline,sapohl/data-pipeline,nathwill/data-pipeline,sapohl/data-pipeline,mozilla-services/data-pipeline