BitmapHeapScan uses the read stream API

BitmapHeapScan uses the read stream API

Make Bitmap Heap Scan use the read stream API instead of invoking
ReadBuffer() for each block indicated by the bitmap.

The read stream API handles prefetching, so remove all of the explicit
prefetching from bitmap heap scan code.

Now, heap table AM implements a read stream callback which uses the
bitmap iterator to return the next required block to the read stream
code.

Tomas Vondra conducted extensive regression testing of this feature.
Andres Freund, Thomas Munro, and I analyzed regressions and Thomas Munro
patched the read stream API.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Tomas Vondra <tomas@vondra.me>
Tested-by: Tomas Vondra <tomas@vondra.me>
Tested-by: Andres Freund <andres@anarazel.de>
Tested-by: Thomas Munro <thomas.munro@gmail.com>
Tested-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/flat/CAAKRu_ZwCwWFeL_H3ia26bP2e7HiKLWt0ZmGXPVwPO6uXq0vaA%40mail.gmail.com