aio: Basic read_stream adjustments for real AIO

aio: Basic read_stream adjustments for real AIO

Adapt the read stream logic for real AIO:
- If AIO is enabled, we shouldn't issue advice, but if it isn't, we should
  continue issuing advice
- AIO benefits from reading ahead with direct IO
- If effective_io_concurrency=0, pass READ_BUFFERS_SYNCHRONOUSLY to
  StartReadBuffers() to ensure synchronous IO execution

There are further improvements we should consider:

- While in read_stream_look_ahead(), we can use AIO batch submission mode for
  increased efficiency. That however requires care to avoid deadlocks and thus
  done separately.
- It can be beneficial to defer starting new IOs until we can issue multiple
  IOs at once. That however requires non-trivial heuristics to decide when to
  do so.

Reviewed-by: Noah Misch <noah@leadboat.com>
Co-authored-by: Andres Freund <andres@anarazel.de>
Co-authored-by: Thomas Munro <thomas.munro@gmail.com>