Supported operations per scope

[adammck]

I recently encountered a system which supports the equivalent of range moves and splits, but not range joins, because (for unimportant reasons) the former was easy to implement, and the latter will be hard. As such we should probably allow each operation to be enabled/disabled on a per-keyspace basis.

Clients are already free of course to reject joins (by considering how many range parents the placement has; more than one means it's a join) and splits (by checking whether the key range of the placement is a subset of its parent range), but there's no way to signal back to the controller that it isn't supported, only that it failed, so it will be retried forever.

This would presumably use the same configuration interface as #14.