Differentiate read- and write-clusters

The ability to place a cluster into "write-only mode" is intended to facilitate operational maneuvers, like re-provisioning a farm. The intended workflow is:

 - Add a new cluster, in write-only mode
 - Perform a backfill, to populate the new cluster
 - Perform sanity checks, e.g. equal key counts
 - Switch the cluster to normal read+write mode

The new mode prevents farm read strategies from querying what's effectively a known-bad set of instances while they're spinning up.