Clean up global connection pooling

[beckjake]

Feature

We should clean up global connection pooling and instead attach any connection pool to the adapter object itself.

Feature description

We currently store the connection pool in a global and it should instead be part of the adapter itself. This should make a client/server model for dbt a lot more sustainable, as potentially a server would have multiple adapters, each with its own configuration and therefore connection pool.

This requires #961.

Who will this benefit?

dbt developers

[beckjake]

Implemented in #1109