Support multiple DBs in a single commit graph

[zachmu]

Today, every database (in the CREATE DATABASE sense) is its own dolt repository with its own commit history. This is the simplest model to understand and implement.

However, some customers may also want multiple logical DBs to share a single commit history. There should be configuration that makes it possible for CREATE DATABASE statements to create a new DB in the same repository, with the same commit history.

[zachmu]

This is now technically feasible with the addition of schema elements to serialized table names.