nixos/sks: Add option to configure database settings

[etu]

Motivation for this change

This can be used for options to tweak the behavior around the database.

For example, I've set up the sks-server on my VPS, 3 weeks later it ran out of disk. Even though it didn't even talk to another server and had just like 3 keys uploaded.

And that was quite annoying but could be solved by adding:

set_flags		DB_LOG_AUTOREMOVE

To the DB_CONFIG files, then it stopped eating ~20MiB disk per day while idle.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[etu]

@grahamc Updated with a fixup, I can squash it later before merging. Just want to keep history for now.

[primeos]

FYI: This broke the module due to a typo (see 753e1e0).

Not to blame anyone (these things happen occasionally) but just as a friendly reminder to test things (I'm assuming both files existed during testing due to a previous mistake) and cc the maintainers (the latter can be quite useful, at least if they actually maintain the package/module) :)

[etu]

@primeos Hey, could you make a backport of that to 19.03 to unbreak the module? That would be great. Sorry for that typo. I thought I tested it well locally because I set it up on my local machine etc. But it appears not.

[primeos]

@etu Yes, had to wait for today though because I couldn't test it yesterday in a VM (without an existing DB, i.e. with a clean setup).

It's now backported in 47e9779 and 0943e4a (turns out there was a second problem with a clean setup - and I've included the full path in the error message to make it easier to find the file).

It's probably not optimal (the module is a bit hacky in general) but it should work now.