Always specify directories for new partition registrations

When creating partition registrations directories must always be defined.

If creating a partition from a PartitionRecord or PartitionChangeRecord from an older version that does not support directory assignments, then DirectoryId.MIGRATING is assumed.

If creating a new partition, or triggering a change in assignment, DirectoryId.UNASSIGNED should be specified, unless the target broker has a single online directory registered, in which case the replica should be assigned directly to that single directory.