Duplicate eviction does not consider scope/collection

This issue only affects deployments that using the scopes/collections feature from Couchbase 7+.

The code in ElasticsearchWriter.write() that detects duplicate document IDs in the write buffer only considers the document ID when looking for duplicates. Two documents may be considered duplicates even if they are destined for different indexes because they come from different collections.

Fix the detection algorithm to consider the destination index as well as the document ID.