WIP exploring better migration

[phewitt]

This isn't ready, but the gist is if we want to use Xandra.Cluster I think we need to not create a new cluster for every setup call. Otherwise we end up with MANY "zombie" pools just wasting load on our DB. Especially for our use case where we also call these setup calls at runtime in some cases.

The goals:

• migrations work alright even when a few nodes are invalid in xandra nodes list
• load is distributed across nodes rather than all blowing up the first node
• we reuse existing connections/pools so we don't needlessly make many connections/pools