Update citation counts from s3

This MR adds a start up task to ingest citation_update.json files from S3 and thus update the citation counts in Elasticsearch. It also updates the `s3-ingests task to carry out citation count updates after metadata ingestion.

This relies on citation_update.json files in the following format:

{"message": [["set", "10.1109/INTMAG.1998.742214", "is-cited-by-count", 100]]}