Stores documents in Google Cloud Storage.
For each pair id, document_text the name of the blob will be {prefix}/{id}
stored in plain text format.
Bucket where the documents will be stored.
Prefix that is prepended to all document names.
Stores a series of documents using each keys.
Gets a batch of documents by id.
The default implementation only loops get_by_id.
Subclasses that have faster ways to retrieve data by batch should implement
this method.
Deletes a batch of documents by id.
Yields the keys present in the storage.
