Refactoring: Centralize SQL connection handling into central crate

[mavilein]

Currently we have a lot of code duplication between the differnt SQL connectors that we have. We should create a new crate in our libs folder that compose prisma-query and all database drivers into a neat abstraction that can be used in all of our SQL connectors.

@do4gr and I disovered that when implementing the SQL introspection tests that required us to copy a ton of code from the SQL migration connector.

[mavilein]

This is something that @tomhoule could tackle.

[tomhoule]

Depending on when we tackle that problem, we may want to do it more or less at the same time as the move to asynchronous database access. prisma-query has lots of breaking changes in the async-await branch, so it will force us to rewrite the SQL connection handling anyway.

[tomhoule]

This is already partially done in master. Once #162 is merged, I think we can consider this issue closed.