Move POI and reports stream persistence logic into Store struct

[neysofu]

First step in moving all store-related logic into a single module to make logic clearer. No functional changes.

Depends on #11.

[neysofu]

All commits after 05ccd79 go towards closing #15.

[Jannis]

I'll review at a higher level than usual.

I've managed to rebase this branch on top of main. I've pushed the rebase here for reference: https://github.com/edgeandnode/graph-ixi/tree/jannis/refactor-store but I didn't want to overwrite anything in this PR just yet.

There are things I like in this PR and things I don't like. I like the store refactor itself, I think it's clean enough.

What I don't like:

• SubgraphDeployment becoming a struct with network information on it.
• POI debug information should only be pulled in when there are differences in POIs but not for every POI ever, that's too expensive. What I was thinking is that this debug information was stored in the db separately from POIs and that it would be linked from POIReports that have a diverging block.

[neysofu]

Understood, thank you for the thorough review of this and all the other PRs. It's been a while so I'd need to play around with it to regain the necessary context... and yeah, good point about only storing POIs differences. I hadn't thought about the explosion of space requirements.

I think we might be able to sync on this once you get back and block oracle won't be so taxing on my schedule.