DoParallelQueries translates between our interface for query batching, and indexes that don't yet support batching.

EnsureDirectory makes sure directory is there, if not creates it if not.

GetParallelChunks fetches chunks in parallel (up to maxParallel).

QueryFilter wraps a callback to ensure the results are filtered correctly; useful for the cache and Bigtable backend, which only ever fetches the whole row.

QueryParallelism is the maximum number of subqueries run in parallel per higher-level query.

ReadCloserWithContextCancelFunc helps with cancelling the context when closing a ReadCloser.

DoSingleQuery is the interface for indexes that don't support batching yet.