[CF 5619] v3 - Read stream scalability improvements and AIO-compatibility

This branch was automatically generated by a robot using patches from an
email thread registered at:

https://commitfest.postgresql.org/patch/5619

The branch will be overwritten each time a new patch version is posted to
the thread, and also periodically to check for bitrot caused by changes
on the master branch.

Patch(es): https://www.postgresql.org/message-id/CA+hUKGKH2mwj=mpCobC4ZyBdR4ToW2hpQcYFDivj40fxf4yQow@mail.gmail.com
Author(s): Thomas Munro

Author: Commitfest Bot