We need a separate transition kind for intra node migration so that we
don't have to recover this information from replica set in an
expensive way. This information is needed in the hot path - in
effective_replicaiton_map, to not return the pending tablet replica to
the coordinator. From its perspective, replica set is not
transitional.
The transition will also be used to alter the behavior of the
sharder. When not in intra-node migration, the sharder should
advertise the shard which is either in the previous or next replica
set. During intra-node migration, that's not possible as there may be
two such shards. So it will return the shard according to the current
read selector.
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