Create adapters as singleton objects instead of classes

[beckjake]

Feature

Instead of being used as bare classes, adapters should be instantiated as singleton objects at run start-time.

Feature description

With adapters being instantiated as objects, we could attach connections to the object instead of in global state. We would also store the configuration on there, and not need the database wrapper logic we currently have, and not need to pass around profile/project information throughout the code. Ideally we would reach the point of not even needing a true singleton, and instead pass an adapter object around to anything that needs it - but that is a much larger task that we can reasonably handle separately.

Depends on #953 and #960

Who will this benefit?

Developers, developers, developers! This is fairly foundational for making adapters cache table metadata, as we need a reasonable place to store the cache. And in the further future, this will make it a lot easier to establish an adapters-as-plugins model.