Add unique index for code column

[fulldecent]

This adds a unique database key to quickly lookup certificates.

Thank you to @Raza403 for contributing his work to this fix.

Fixes #666.

[fulldecent]

@mdjnelson for your review, please

[fulldecent]

Hi @mdjnelson could you please approve to run the workflow?

[mdjnelson]

Done. The only issue with me merging is the upgrade path. The number needs to be different for each supported version. I will do this myself, no worries but need to set some time aside to go over a few PRs.

[fulldecent]

Thank you, much appreciated.

If there's anyway that I can make your job easier next time, please leave some notes in the README or CONTRIBUTING file.

I have another PR here and planning to work on an additional one after that.

[mdjnelson]

Putting this in now. The version bumps are because we dont want to bump all the branches to the same version, as there may be upgrades, say for example, just for 4.2. If someone upgrades to the same version on 4.1 then updates their code to use 4.2 they are going to miss those upgrade steps necessary in 4.2

[mdjnelson]

All done, thanks.