Add session config for local rack and local dc

[Lorak-mmk]

Currently setting local DC and locak rack for DefaultPolicy is unergonomic. User needs to create execution profile, create policy (with local dc/rack set), and set the default execution profile on session builder.

Better way would be to have a method on SessionBuilder to set it. Then LBP could retrieve it from RoutingInfo / ClusterState.
Configuring it this way would also help with adding new parts of the driver that need to know local DC, like #1279