[CF 5633] v6 - Lock-free XLog Reservation from WAL

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

https://commitfest.postgresql.org/patch/5633

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/7b0d7b56-d3a2-485b-84fe-a5951004715b@postgrespro.ru
Author(s): Yura Sokolov

Author: Commitfest Bot