Add base implementation for missing handlers

Dear @JeroenDeDauw, @brightbyte, @filbertkm, @lydiapintscher,

this is split from #51 and #54. I wonder why, on one hand, everybody is complaining about simple queries being stuck, but on the other hand the most basic patches are rotting for almost a month now. I mostly blame GitHub. I tried to continue working on the (unmerged) data value handlers and keep them up to date. But there is no mental model for that in GitHub. Updating unmerged pull requests requires everybody to review them over and over again with less and less progress. This turned into an endless spiral down weeks ago and got really silly recently. At least one of my updates got reverted (I noticed one revert, no idea what else got lost) because of GitHub's stupid use of "force push". I still have no idea why it does that. It's like in The Boy Who Cried Wolf. The `-f` gets completely meaningless if you are constantly forced to use it for no good reason.

I have no idea how to continue. I'm not even sure if this pull request here is a good idea since it does not solve the original problem that GitHub misses a model for changes that depend on each other. Plus most people not reading the spam-like emails from GitHub. I'm close to abandoning all my GitHub patches.