[chore] move caches to a separate State{} structure

[NyaaaWhatsUpDoc]

This moves toward consolidating much of our means of dependency injection. It will also make it easier sharing resources across the codebase, and in the particular case of the caches, invalidating the necessary caches on particular changes.

[tsmethurst]

looks like it's going in a good direction!

is it worth making state.State into an interface instead of just using the struct, so that in future we can use different kinds of state objects? (just thinking years in the future here, when we start messing about with remote state etc)

[NyaaaWhatsUpDoc]

@tsmethurst I started to make the State{} itself an interface, but it complicates being able to be granular about initiating individual parts without having tonnes of interface methods. Instead what I went for was making each individual attached piece being an interface. The DB obviously already is, so I updated the caches to now be interfaces which could allow swapping them out for a multi-node compatible deployment in the future.

I would say this is ready to review now, but getting this 0.6.0 release out is more important first for sure :D

[tsmethurst]

lmk when you want me to review this again kimbe :)

[tsmethurst]

I had a few comments but nothing significant. This looks really great, and is good to go afaict :) Amazing work kimbe