new parameter keySuffix-- If not null, keySuffix is appended to the lookup key when loading and storing digest history. Thus the key looks like {digest}{keySuffix}, e.g.  "sha1:22SFHXERHNFOEY6WK7YOUN4PFIPZSB4D-1193". The purpose is to support multiple namespaces in a single hbase table, to avoid proliferation of small tables. The reason we use a suffix instead of a prefix is to leave open the possibility of deduplication across these different namespaces at some point in the future.
only write WARC-Refers-To header when value is not null