Geo: Preliminary research on what is needed for PostgreSQL 12

Background

There was some conversation in the #gdk channel about using PG12 with Geo. The discussion started, because Ubuntu 20.04 ships with PG12, see the GDK issue for context: gitlab-development-kit#682 (comment 335540308)

One change in PG12 might be relevant/important for Geo (from the release notes):

Move recovery.conf settings into postgresql.conf (Masao Fujii, Simon Riggs, Abhijit Menon-Sen, Sergei Kornilov)

recovery.conf is no longer used, and the server will not start if that file exists. recovery.signal and standby.signal files are now used to switch into non-primary mode. The trigger_file setting has been renamed to promote_trigger_file. The standby_mode setting has been removed.

@eread was testing Geo on GDK and PG12 a little while ago, but he ran into this issue: https://pgstef.github.io/2018/11/26/postgresql12_preview_recovery_conf_disappears.html

The article dates back from back from before PG12 was released, so some info might be incorrect. But still, there might a problem.

Impact on GDK

It's not clear to me what is wrong with PG12 on GDK. I guess we should set it up, and see what happens. I'm hopeful we can fix it with minimal changes.

Impact on omnibus

This is even less clear. @stanhu thought recovery.conf was used to promote a secondary, but it seems pg_ctl promote is used, so we should be good here

Suggestion

We better try to make GDK work with Geo and PG12 soon, but increasingly more people will no longer be able to run PG11 on their development machines.

Also PG12 will be landing in 13.3, so it would be nice to be ahead of the game.