Enable PostgreSQL backend and fix libspatialindex branch name.

[stateisbad]

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• I have read the contributing guideline and understand that I have made the correct modifications

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Description:

Adds support for the PostgreSQL storage backend, as well as fixes the libspatialindex primary branch name (it is now main)

Benefits of this PR and context:

The default backend, SQLite, performs well on smaller maps, but on servers where the map is quite large, it begins to slow down considerably. PostgreSQL is fully supported by Minetest, and offers the best performance in such cases.

How Has This Been Tested?

This has been tested by building a local image from this branch and running in a lab environment along side the official postgresql image, both with brand-new worlds as well as using the minetestserver --migrate command to migrate an existing world.

Source / References:

https://wiki.minetest.net/Database_backends

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[stateisbad]

Not sure what's up with the CI failure, as it's pretty opaque. The changes are minimal, so if the image builds, it should run.

[thespad]

It's timing out waiting for the init to complete (which in this case is a successful UDP connection on port 30000).

Seems something has changed either upstream or within Alpine and localhost is resolving to the ipv6 address, but minetest is only bound to the IPv4 interface.

As a temporary fix you can edit etc/s6-overlay/s6-rc.d/svc-minetest/run and change

"nc -zu localhost 30000"

to

"nc -zu 127.0.0.1 30000"

[thespad]

Turns out there was a bunch of work required to get the init check working and also some other maintenance that was necessary on the image so I've incorporated your changes into #60 which also addresses the CI issues you were seeing.