feat!: Move Caching into Equinox; remove Store -> Core deps

[bartelink]

Only 99% sure this is for the best as:

• it doubles the perceived lines of code in Equinox
• major next/final step is to push out Log dependency, and maybe that's easier to do s part of feat(MessageDb): Replace logging with complete OT metrics #407 without doing this first? (Having said that, moving the Log dependency out to the stores is orthogonal to whether Caching lives in Equinox)

But ew also have some benefits:

• if you want to understand the caching, you need to fish it out of the Async.Core pile
• Equinox.Core now becomes a freestanding lib that a) does not depend on Equinox b) is not depended on my the Stores
• we can now version Equinox (single thing that all stores depend on and) and Equinox.Core (grab bag of stuff that only Equinox.Core uses AsyncCacheCell out of) as independent beasts. Then one could have a store impl version and an Equinox version that vary independently. (That would be a small change to the MinVer tagging config, and would make the current NuGet upload logic fail (e.g. if you build N packages but only one is new, the upload stops at the first one in alphabetical that's a duplicate) (but we don't use it anyway atm so it's also moot))