arrowbricks v5.1.2

• Stop the background fetch as soon as a result is abandoned. Cancelling
  the task that awaits a fetch, closing a stream_query_json iterator
  (what a web framework does when the client disconnects), or dropping a
  Cursor before its result is drained used to leave the chunk downloads
  already in flight running to completion (up to chunk_fetch_concurrency
  of them), and workers could start new ones after the consumer was gone.
  In-flight downloads are now aborted right away, on both protocols.
• Thrift: closing the operation after an abandoned fetch no longer hangs.
  With more links left than the download workers and their queue could hold
  (about 2 * chunk_fetch_concurrency; 128 by default), the fetch loop
  blocked forever, so CloseOperation (and CloseSession for a throwaway
  session) never went out, and the task, its client handle and its
  connection pool were never freed. The loop now also stops issuing
  FetchResults once nobody reads the result.
• Unchanged: server-side cancel (CancelOperation / SEA POST .../cancel)
  still fires when a still-running statement is abandoned, as since 3.2.0.
• Tests: wiremock_thrift.rs/wiremock_pipeline.rs drop a partly read
  result with a download in flight, and test_thrift_pipeline.py stops
  reading stream_query_json after the first rows. All three fail on 5.1.1.
• No API changes.