[Store 5] Make Bookkeeper optional

[matt-ramotar]

As I wrote internally - I am not sure we need to require users pass in Bookkeeper. All Bookkeper needs to do is let RealMarket know it should eagerly resolve conflicts. Initially I thought we would want to defer implementation to users. But with Bookkeeping mental model, I suspect most users will not care to customize. We can bind Bookkeeper to a disk cache or DB ourselves under the hood and make it default implementation. Thoughts on making Bookkeeper optional?

Originally posted by @matt-ramotar in #450 (comment)

[aclassen]

Just wanna note, that if we wanna go for disk cache we would also need a kmp filesystem implementation. So i guess a DB would be a better fit to support all platforms.

[matt-ramotar]

Closing as #496 separated Store and MutableStore