[core] Implement Conn managing DB metadata, transacting, and SQLite threading

[ncalexan]

This issue tracks implementing the start of the Conn abstraction. A Mentat connection is the conceptual equivalent of a SQLite connection. One crucial difference is that Mentat enforces a single writer and handles some isolation. This is similar to Datomic's Java Connection API.

@rnewman (and I) have been thinking at some length about how to represent the structure in Rust; he's written about this at https://github.com/mozilla/mentat/wiki/Thoughts:-modeling-db-conn-in-Rust and more he shared some notes at https://public.etherpad-mozilla.org/p/db-conn-thoughts.

Getting some pieces of this correct up front will help guide a lot of the abstractions we're building -- see comments like #283 (review) and friends.

[ncalexan]

This is not a good first or second bug.

[ncalexan]

I think @rnewman's review in #283 will be easier if I sketch some of this out in code.