Support backfill deduplication in BigQueryIO.write()

See https://github.com/GoogleCloudPlatform/DataflowJavaSDK/issues/603 motivated by SO question https://stackoverflow.com/questions/46076914/apache-beam-update-bigquery-table-row-with-bigqueryio

Perhaps one way we can do this is make BigQueryIO return a PValue that can be sequenced with other things, and implement a BigQuery.update() transform that executes a single DML statement (or a small collection thereof - since DML in BigQuery is very scarce), and let the user sandwich them together if they would like to.

Imported from Jira [BEAM-2864](https://issues.apache.org/jira/browse/BEAM-2864). Original Jira may contain additional context.
Reported by: jkff.

Start with BigQueryIO.write() and read the linked DataflowJavaSDK issue 603, Stack Overflow question, and Jira BEAM-2864 for context. Define the desired backfill-deduplication behavior and assess the proposed sequenced PValue/BigQuery.update() approach; done means users can update BigQuery table rows through the supported transform without duplicate backfill results.

Written by the indexing model from the issue text.