Support for Postgres-like Clusters

[robd003]

I'm using CrateDB which provides a Postgres interface and allows you to run a cluster of nodes that can all be used for queries. It would be great to have a way to distribute queries across nodes and be able to safely recover if one nodes goes down but the rest are available.

It would be great to be able to just define multiple Postgres endpoints and have asyncpg round-robin non-transaction queries to them.