Implement immediate persistence (possibly with gokv)

[codefromthecrypt]

I ran into this comment on langchaingo and while hinted on the roadmap, might be nice to have an issue to subscribe to.

Perhaps an experimental implementation could be co-raised as a PR here and a draft PR on langchaingo for integration.

[philippgille]

• Regarding gokv integration: I'd prefer to wait for the next version, in which we want to add the typical context.Context param to the Set/Get/Delete methods. There is some ongoing work already in that direction.
• Regarding langchain-go integration (without gokv support): I actually have a draft PR locally with the implementation itself and example code (in their example directory), but the unit tests are still missing.

I think both integrations can be implemented separately at first, and later they can be combined. I don't want to tie people to any specific implementation, so the way I envision this is to (re-)declare the gokv interface here in chromem-go (to avoid the dependency on gokv), and have a constructor similar to the current NewPersistentDB, where users can inject whichever gokv implementation they want to use, or provide their own implementation of the simple interface. And in langchain-go we can then also add a separate constructor which calls this aforementioned chromem-go constructor, so users can in the same way inject any gokv implementation they want to use.

I'll update this thread when there's progress on one of the two points.