Query for the latest versions of multiple documents

This endpoint must be called when querying the latest versions for multiple documents. It's an aggregate endpoint that should make periodic polling of a collection of documents easier. Servers must also support the ETag header for this endpoint, so clients may specifiy to receive a response object in case of changes on the server since the last query. It will return a list of DocumentVersion objects, where each DocumentVersion represents the latest version for each given document on the server.